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VOL. XXXV. PART III.— (No. 19)— FOR THE SESSION 1887-88. 



No. XIX. On the Development and Life Histories of the Teleostean Food- and other Fishes. 
By Professor W. C. M'Intosh, F.R.S., and E. E. Prince, B.A., St Andrews 
Marine Laboratory. (Plates I. to XXVIII.), . . . . 665 

(Issued 3rd February 1890). 

( 665 ) 

XIX. — On the Development and Life- Histories of the Teleostean Food- and other Fishes. 
By Professor W. C. M'Intosh, F.R.S., and E. E. Prince, B.A., St Andrews Marine 
Laboratory.* (Plates I. to XXVIII.) 

(Read 18th June 1888.) 

For Table of Contents see end of paper. 

I. General Remarks. 

Until very recently existing information concerning the eggs and oviposition of 
British fishes, and more especially marine fishes, was of the most fragmentary 
character. In the standard works upon Ichthyology, such as Owen's Anatomy of 
Vertebrates (vol. i. Fishes), it is comprised in a few vague sentences ; while the original 
papers published by British ichthyologists are not numerous, and refer, for the most 
part, to fresh-water species. Within the last few years, however, attention has been 
more systematically directed to the subject, and the enlightened views of the late Royal 
Commission on Trawling, and more especially of its chairman, the late Earl of 
Dalhousie, has given a fresh impetus to the study of the development and life-history 
of our food-fishes, as preliminary to a thorough investigation of their habits, food, so- 
called migrations, and general life-history. 

The following paper comprises the first results of our recent work at the St Andrews 
Marine Laboratory. 

Though much has been done by foreign observers of late years in regard to the 
development of marine fishes, yet the cod and herring only, amongst those conspicuous 
by their economic value in this country, have been specially dealt with. 

It was therefore necessary, even at the risk of repeating some observations already 
known to science, to examine as thoroughly as possible the ovarian growth, oviposition, 
hatching, and development of such of the important white fishes as could be obtained, 
and to fill up the gaps in our knowledge of the period between the escape of the embryo 
from the egg, and the young, though advanced, forms known to naturalists and fishermen. 

* The authors have to acknowledge the courtesy of the Fishery Board for Scotland, under whose auspices the work 
has been accomplished, and to whom all credit is given. Grants from the Royal Society (Government Grant) and 
from the British Association have also been of great service in regard to assistance and apparatus. To Dr Scharff, 
B.Sc, now of the Museum of the Royal College of Science, Dublin, for valued aid of various kinds in 1886, and to 
Dr J. Wilson of St Andrews, for help in making sections, our acknowledgments are also due. When cruising in the 
Fishery Board tender " Garland," Mr W. L. Calderwood, B.Sc, and Mr H. E. Durham, B.A., also kindly gave 
assistance. It may further be stated that the first part of the paper, containing the development of the food-fishes and 
their early larval condition, was mainly the work of Mr Prince ; while the account of the post-larval stages, the 
development of Anarrhichas and the salmon, was the work of Dr M'Intosh. Mr Prince added further notes on the 
structure of the later stages of other forms. 

VOL. XXXV. PART III. (NO. 19). 5 Q 


The ova of about forty British fishes have been examined, and in most cases the 
development of the young before and after leaving the egg, as far as possible, followed. 
The period over which the special observations extended commenced with the work for 
H.M. Trawling Commission in 1884, when the talented chairman (Lord Dalhousie) placed 
every encouragement (personal and administrative), and all the facilities in his power 
for pursuing the subject as thoroughly as time would permit. The experience of former 
years at St Andrews and elsewhere has been made available, especially in regard to the 
growth of marine fishes, and to the structural features in the later stages of the salmon. 

The ova examined at St Andrews may be conveniently arranged in two divisions, 
viz., Pelagic or floating eggs, and Non-Pelagic or demersal eggs. 

Under the former head twenty-three species may be grouped, viz., Long-Rough Dab, 
Turbot, Plaice, Lemon-Dab, Craig-Fluke, Common Dab, Common Flounder, Sole, Muller's 
Topknot, Ling, Five-bearded Rockling, Cod, Haddock, Bib, Whiting, Poor Cod, Green 
Cod, Pollack, Frog-fish, Skulpin, Lesser Weever, Sprat, and Grey Gurnard. Besides the 
foregoing, the Common Eel and the Conger have been examined ; but their pelagic or 
demersal character has not been finally determined. 

The non-pelagic ova include at least fourteen species, besides a few doubtful forms of 
which the ovarian eggs alone have been under consideration. This (demersal) group 
embraces the Herring, Smelt, Salmon, Trout, Bimaculated Sucker, Wolf-fish, Shanny, 
Viviparous Blenny, Montagu's Sucker, Lump-sucker, Goby, Armed Bullhead, Cottus, 
Fifteen-spined and Three-spined Stickleback, Sea Bream, Gunnel, &c, besides the 
Cyclostome — Myxine. Amongst the doubtful eggs are those of Yarrell's Blenny and the 
Sand -Eel (Ammodytes tobianus). 

II. The Mature Ovum. 

General Features. — The mature ovum of osseous fishes is generally of comparatively 
small size, spherical in form, and more or less translucent. Two parts may be distin- 
guished, viz., a protective external capsule (PI. I. figs. 1-4. zr), and a contained vitelline 
mass (y), the latter consisting of a globe of food-yolk, with interfused germinal matter. 
Upon being placed in water, the ova of some species float near the surface and throughout 
the water ; these, as already pointed out, form the pelagic group; while in other species the 
eggs sink to the bottom, and form the second group, viz., the demersal or non-floating eggs. 
The first group exhibit in a striking way the feature characteristic of pelagic structures, 
viz., a colourless translucency ; while the second or demersal group are less delicate in 
appearance, and often tinted in a marked manner. Thus the freshly extruded ova of 
Cyclopterus lumpus are of a brilliant purplish rose, or a subdued green or yellow tint, 
which soon, however, fades away, and the eggs become more translucent. The ova of 
the salmon, by their rich orange colour, afford a familiar example of tinted demersal eggs ; 
while those of many species of Stickleback (e.g., Gastrosteus spinachia) are of a trans- 
parent amber-tint. Such coloration, as just noted, like the whitish opacity of the ovarian 


ovum, may be transient and give place after extrusion to an imperfect transparency. 
The ova of Ammodytes tobianus present a marked example of this, for while contained in 
the ovary they are of a bright orange colour — the ovaries on this account forming a 
bilobed orange-tinted mass in the abdomen of the nearly ripe female, but the eggs when 
ready for extrusion, and indeed while passing to the oviducal aperture, would appear to 
become colourless. Pelagic eggs usually float loosely together or singly, and do not 
adhere to each other, save in certain noticeable instances, of which Lophius piscatorius is 
an interesting example. Agassiz first described the floating eggs of this familiar fish as 
adhering together in long bands near the surface (No. 1, p. 280), but even in this case 
eggs may become detached and float free (No. 2, p. 16). Professor E. van Beneden 
describes some minute isolated and agglutinated eggs which he was not able to deter- 
mine, but believed that they belonged to a species of Lota, and he supposed that, after 
being deposited in a mass, they " remain for some time adherent one to another, and 
afterwards separate, and then float free from all adhesion, on the surface of the sea " 
(No. 25, p. 41). This surmise is perhaps questionable, and Van Beneden, indeed, himself 
adds — " I never saw the eggs become detached from one another" (p. 42); and they prob- 
ably, therefore, belonged to two different species. Eggs similar to those of E. van Beneden 
were obtained by Haeckel on the coast of Corsica. They formed agglutinated masses of 
various volume and form — the ova being in fact imbedded in a gelatinous substance. * 
Pelagic ova, if ever adherent, possibly may soon become detached, but eggs deposited on 
the sea-bottom, in masses, adhere together most strongly, though in an advanced stage 
they are less firmly united, this loss of adherent property in such a form as Cyclopterus 
lumpus taking place only after the lapse of a considerable interval, often many weeks, 
when the capsule becomes softened, and changes occur in its physical character, probably 
to facilitate the liberation of the contained embryo. Usually, however, these eggs cling 
together if undisturbed (even when dead) for long periods. The adhesive character 
which Von Baer was the first to notice in certain Cyprinoidst is due to a mucilaginous 
ovarian secretion bathing the eggs, and acting as a lubricant during extrusion. On 
exposure to water, it has the property of hardening, as in many similar instances both in 
vertebrates and invertebrates ; and, in the case of adherent eggs, it acts as a cement, bind- 
ing them together so firmly that they can be separated only with difficulty ; and the points 
where the adjacent eggs were in contact show prominent scars or facets after separation 
(PI. I. figs. 2, 3, and 4, x). 

A marked translucency of both capsule and egg-contents usually indicates the healthy 

* Mr Rattray has recently submitted to us examples of pelagic ova from the west coast of Africa, which are also 
bound in masses by a connecting substance converted by reagents and alcohol into a thread-like meshwork. Threads 
of a like character were noticed in some ova sent by M. Millet many years ago to the French Academy of Sciences. 
They were evidently demersal eggs, for they were attached to a wooden barrel hoop by the elastic threads, the latter 
forming a felted meshwork, which Millet supposed to be produced by the parent-fish (No. 110, p. 342). They were 
procured in 14° 15' N. lat. and 20° 30' W. long. The eggs Mr Rattray kindly sent to St Andrews were obtained (in the 
s.s. " Buccaneer " Expedition) in lat. 1° 17' 6" N., long. 13° 54' 4" W. Vide Remarks on these by Mr J. T. Cunningham, 
Trans. Roy. Soc. Edin., xxxiii. i. p. 108, pi. vii. fig. 7. 

+ Untersuchuncjen iiber die Entvjickelungsgeschichte der Fische, Leipzig, 1835, p. 7. 


living egg, especially in the case of pelagic ova, and also to a certain extent in demersal 
forms. This translucency is due to the disappearance of the granules in the yolk of the 
ovarian egg when ripe. Sometimes, however, eggs which are not perfectly mature, i.e., 
lack the translucency of the ripe ovum, may yet be fertilised, and their embryos in due 
time liberated. This was frequently the case with imperfectly ripe eggs of T. gurnardus, 
which, though presenting slight opacity, were successfully hatched. Occasionally eggs of 
the species just named exhibit a remarkable pinkish or reddish coloration, the oil-globule 
being of a dark tint (PI. XVI. fig. 10). The cause or meaning of this abnormal appear- 
ance is undecided; the eggs, of course, were not fertilised, and did not develop ; indeed, 
this coloration has only been seen in dead eggs. Pelagic eggs, when dead or unhealthy, 
show a great increase in the perivitelline space, and sink to the bottom of the tanks. 
Sometimes living eggs, from various causes, such as a change in the specific gravity of 
the water, sink, this being frequently the case with T. gurnardus ; yet when the water 
is violently stirred, or when removed from still water for examination, and then emptied 
into the tanks, they again often assume their buoyancy. This may be due to the dis- 
engagement of particles of foreign matter, such as sand, though this is not always 
evident. The eggs of Molva vulgaris (PI. I. fig. 10) are less buoyant than some other 
Gadoids, e.g., Gadus morrhua and G. ceglejinus, and sometimes, though living, sink to 
the bottom in quiescent water, yet successfully develop. The ova of the ling are indeed 
more delicate, and more susceptible to unfavourable conditions than those of the cod and 
haddock. The addition of spirit to a vessel containing them causes them to rush to the 
side of the vessel, and cling to it with tenacity. The hardy character of certain pelagic 
eggs and their vitalit} 7 was shown in many cases at the Laboratory. No difficulty was 
found in developing eggs fertilised at sea and conveyed long distances, in some cases after 
travelling in earthenware jars for three or four days. Eggs of the cod contained in such 
jars, three-fourths filled with sea- water, reached the Laboratory on the fourth day after 
fertilisation, and though most of the eggs had sunk to the bottom, and the w T ater was 
offensive with putrid matter — Infusoria, Bacteria, and Spirilla being abundant, yet many 
of the eggs still floated at the surface, and the hearts of the embryos pulsated regularly. 
The effect of cold is to retard development, but is not detrimental unless extreme. In 
one instance a series of the eggs of the haddock were floating buoyantly in the tanks at 
6 p.m., but next morning the glass vessel was covered with a coating of ice, on breaking 
which most of the eggs fell to the bottom, and in these the yolk and germinal area were 
found to be much shrunken and corrugated, leaving a wide space round the vitelline 
mass. A few only survived, these having apparently remained under the trickle of the 
supply pipe.* That pelagic eggs float in sea water, while they sink in fresh water, or in 
sea water having an admixture of fresh, Professor Baird has shown to be due to the 
fact that their specific gravity is about 1'020 or l*025.t 

* Vide Nature, June 1886. 

t Of this floating property, the oldest fishermen, Baird adds, had not the slightest idea ; they thought " that the 
females deposited their eggs on the rocks, where they were visited and impregnated by the males They had 


Pelagic Ova. 

General Remarks. — The pelagic nature of the ova of so large a number of valuable 
food-fishes removes them altogether from many of the vicissitudes which befall demersal 
eggs. Their transparent glassy nature, minute size, and enormous abundance, sufficiently 
provide for their safety and the increase of the species. Pelagic ova are by no means 
common in the stomachs of fishes, while ova deposited on the bottom {e.g., those of 
Cyclopterus, Cottus, and Clupea harengus) are eaten by many fishes with great avidity, 
yet the numbers of one of these at least are, so far as can be made out, by no means 
seriously affected. How much more surely, then, is the multiplication of those with 
pelagic ova provided for ? As a rule, they are deep enough to escape the vicissitudes of 
the immediate surface, and in our country are seldom stranded on the beach in numbers 
sufficient to attract attention.* The larvae which escape from them are also minute and 
translucent, and thus are less prone to attract the notice of predatory marine forms ; more- 
over, they soon become very active, while their purely pelagic life gives them a vast area 
for their safe development. 

The contrast between such types and the condition, for instance, in Cottus, is marked. 
In the latter the ova are deposited between tide-marks in masses, and are often devoured 
by other fishes, and it may be by predatory birds and mollusks. The comparatively large 
young are conspicuous objects, and can only escape by keeping within reach of tangles 
and other sea-weeds, a constant reduction of their numbers taking place, notwithstanding 
their defensive armature, during the somewhat slow growth to the adult condition. It is 
possible, indeed, that though the egg-capsules in Cottus are much denser, and the embryos 
larger and more highly developed than in the cod, a much greater number of the latter 
proportionally reach maturity than in the case of the former. 

On the eastern shores pelagic ova begin to appear at the end of February, though 
there is no reason why some should not be found earlier, as Dr J. Murray tells us they 
are on the west coast (Clyde district), and a kind of succession of those of different species 
occurs throughout the spring, summer, and autumn. Amongst the earliest are the ova 
of the plaice, Motella, and the large egg with the spacious peri vitelline space, the larval 
form issuing from which is described subsequently. Those of the Gadoids, such as the cod, 
haddock, and whiting, next appear, and also those of the flounder and dab, while towards 
the end of the month the eggs of the gurnard are also captured. April is characterised 
by the abundance of pelagic ova, the maximum perhaps being attained towards the latter 
part of the month, when the ova of the sprat t and other forms swell the list. As an 

at times noticed the little transparent globular bodies in the water; but it never occurred to them that they were the 
eggs of any fish. They may be found at the surface in common with the eggs of pollack, haddock, and probably other 
species of the cod family, when the sea is smooth, but when the water becomes rough they are carried to a depth of 
several fathoms by the current, though the tendency is to remain near the surface " (No. 8, p. 715). 

* G. 0. Sars found, however, that they were so at Lofoten. 

t Hensen first noticed the pelagic ova of the sprat, and his observation has been corroborated by J. T. Cunning- 
ham and ourselves. Other Clupeoids, as shown by Raffaele, also have pelagic eggs. 


example of the duration of a particular kind of ova in the bay, those of the gurnard may 
be taken ; for, appearing in April, they continue throughout May, June, and even part of 
July, being very abundant in June. It is clear, therefore, that with rapid growth, the 
differences in size between the post-larval forms produced from the ova at the extremities 
of the period must be considerable. 

Distribution. — Nothing was more striking, in the investigations in connection with 
H.M. Trawling Commission in 1884, than the abundance of the pelagic ova in the upper 
regions of the water, and indeed throughout it. They are not usually found quite at the 
surface, but as soon as the tow-net is sunk a fathom or two, they occur almost in every 
haul on suitable ground. Though on the banks frequented by the cod, haddock, and 
whiting, these pelagic ova are in greatest profusion at the breeding season, yet they are 
met with during many months from January till late in autumn, a continuous discharge 
of ova taking place from one or other group having this habit. Moreover, it is clear that 
the provision by which only a portion of the ovary in most fishes with pelagic eggs becomes 
ripe at a given time, greatly prolongs the spawning period, and tends to intensify the 
feature just mentioned. It is possible indeed to form an estimate of the number of 
spawning fishes in a given district by the abundance of pelagic ova, or the contrary. It 
is only necessary to illustrate this by reference to the surface of Smith Bank, off the coast 
of Caithness, where the ova and embryos were in vast numbers in the beginning of April, 
so much so that the area resembled a vast hatching-pond, even the sea-birds feeding in 
long lines on such as the currents swept to the surface. The same feature was shortly 
afterwards noticed, along with Lord Dalhousie, off the Island of May, though both eggs 
and embryos were less numerous than in the former case. 

Again, recent investigations with the trawl-like tow-net on the bottom show that a 
vast number of pelagic ova, such as those of the cod, whiting, rockling, sole, flounder, 
gurnard, sprat, and other forms, are to be found there — when the large mid- water net and 
the surface-net are nearly devoid of them. Whether this aggregation of ova is due to 
cold at the surface or to the effect of currents has not yet been determined, but it is a 
feature of great interest. 

Sizes of Ova.* — As an example of the variety of pelagic ova common to the sea 
beyond the Firth of Forth in April, the following measurements from spirit-preparations 
are interesting. The ova were collected by the tow-net (sunk a fathom or thereabout) 
in the usual manner, and then placed in strong spirit, which caused considerable contraction, 
probably from - 1 to "15. A very few measured '0216 of an inch, others had a diameter 
of -023, -03, -033 (probably Motella), -035, '0366, '04, '043, '045, '046, '05, the largest 
number ranging over the area covered by the last five, which probably included cod, 
haddock, ling, &c, "056, and a very few at *058 and '083 of an inch. A little variation 
appears to occur in each species. The average in fresh specimens of the haddock is 
•056, the blastodisc being '033; plaice, '0716; ling, -0916, and the oil-globule, -031; 

* A table of sizes of ova from Raffaele is given by one of us, in a Report on the Pelagic Fauna of St Andrews 
Bay, Seventh Annual Report, Fishery Board for Scotland, 1889. 


cod, *06 ; grey gurnard, "055, and oil-globule, '0116; lemon dab, '053; flounder, "038 ; 
common dab, "033 ; skulpin, '025 to '030 ; sprat, '044 in long diameter, '039 in short 
diameter ; sole, '045. 

The Egg- Capsule, with Remarks on the Reproductive Organs and 

Period of Spawning. 

Few points in the constitution of the ovum afford more matter for controversy than 
the origin and significance of the external protective membrane. 

The twofold division of egg-membranes, due to Prof. E. van Beneden (No. 24, 
pp. 228-30), and founded upon their derivation, is both natural and convenient, viz., (1) 
membranes differentiated from the cortex of the egg-mass itself; (2) membranes formed 
ab extra by the cells of the ovarian follicle. It is generally agreed that the egg-capsule 
of. Teleostean ova belongs to the first division. Cunningham, however, does not adopt 
this view, and the " vitelline membrane " of his earlier papers he now considers to 
be an extra-vitelline product — developed by the cells of the follicular epithelium 
(No. 47). Other protective structures may lie outside the egg-capsule proper, such as 
the mucous layer in Perca Jluviatilis, the gelatinous matter surrounding the floating ova 
of Lophius piscatorius, and others, but they are probably ovarian, oviducal, or other 
secretions, and do not belong to the ovum proper. Further, it seems most in accord 
with present results to regard the external capsule as a single membrane — variously 
styled Eikapsel (Muller, His, &c), Eihaut (Kupffer), Chorion (Lereboullet), Ectosac 
(Owen), outer yelk-sac (Ransom), and zona radiata (Waldeyer). G. Brook, again, 
describes in Trachinus a thin membrane (his vitelline membrane) outside the zona. 
Such has not been seen in any of our pelagic eggs. It is generally hyaline, tough, and 
slightly resilient, and varies in thickness in different species — thus approximately in 

Anarrhichas lupus, it is '00143 to '00162 in. 

Gastrostcus spinachia, „ '0015 „ 

Gadus morrhua, „ '000312 „ 

„ aiglcfinus, „ '000440 „ 

Gadus merlangw, it is '000310 in. 

Pleuronectes jlesus, „ '000125 „ 

„ limanda, „ '000104 „ 

Trigla gurnarclus, „ "000333 „ 

In pelagic ova it is so exceedingly thin and translucent that the developmental 
changes in the germ are visible through the capsule,* yet in demersal ova it is not only 
denser, but presents in many species marked structural features, such as projecting knobs, 
filamentous processes, reticulations, and the like, all of which, however, must be looked 
upon merely as modifications of the single capsular membrane — the zona radiata. It is 
very thin and transparent in the sprat, the egg of which generally shrivels when put in 
spirit. The zona usually presents laminae, which Sars observed and counted in Gadus 
morrhua ; but such does not imply the existence of separate layers, for chitinous structures 
of this kind often show a stratified condition. Ryder could only make out the laminae 

* In undeveloped and dying eggs the growing opacity of the vitelline mass is readily seen. This opacity of the 
egg-contents Andr£ wrongly attributes to the capsule itself, which he says becomes opaque (No. 4, p. 397). 


in Gadus morrhua after treatment with osmic acid, but in both that and other species 
they were observed at the St Andrews Laboratory without preparation. As the time 
approaches for hatching, the capsule (e.g., in Gadus ceglejinus) often breaks up into 
flakes like the translucent chitinous secretions (tubes) of Annelids. The continued action 
of water and other causes seems to produce this physical change, so that the embryo is 
more readily extruded. 

We shall glance first at a few of the prominent features of demersal ova — the two 
most obvious points as compared with pelagic eggs being (1) the greater density of the 
zona radiata ; (2) the tendency to adhere together in masses by reason of the peculiar 
secretion which issues from the oviduct along with the ova. One of us has pointed out,* 
that in adhering together, eggs such as those of Cottus and Cyclopterus (vide PI. I. 
figs. 1-4) do so by limited areas of their surface, i.e., by facets, and thus the mass of 
ova is traversed by an intricate system of channels, which ensures more perfect aeration 
in the circumstances in which they are placed, e.g., in rock-pools. In the slow-running 
tanks of the Laboratory, however, these eggs develop less successfully than detached 
and floating forms, since the decomposition of a few frequently causes the death of the 
whole mass. 

Considerable variations are presented by the external surface of the zona radiata. 
Thus in Lepadogaster bimacidatus the capsule shows very evident punctures, and the 
ova, instead of being fixed to each other, are attached separately to shells, stones, and 
similar structures. Anarrhichas lupus, again, has the largest non-pelagic egg known 
to us. During the investigations for H.M. Trawling Commission in 1884, one of us 
had been familiar with the ovarian eggs of this form in their earlier stages, and in a 
morbid ovary some of the fully developed eggs were retained so late as the month of 
February, the spawning period apparently extending over the late autumnal or winter 
season, probably from October or November to December. It was not until the 16th of 
January 1886, however, that normal mature ova were obtained. A local trawler pro- 
cured in comparatively shallow water (5 to 6 fathoms) a large mass of them. These ova 
(PI. XX. figs. 6, 7) are of a pale straw colour, with a slight opalescent hue. In shape 
they are more or less spherical, and measure 5 '5 or 6 mm. in diameter. The zona 
radiata presents a comparatively smooth, though minutely punctured appearance 
(PL XX. fig. 8), and is very tough, so that the eggs, which are fixed to each other in 
the usual manner to facilitate aeration, can only be torn asunder with difficulty. In 
section (PI. I. fig. 25) a stratum (a), marked by a deep hsematoxylin-stain, separates an 
outer thicker from an inner thinner portion of the zona radiata. Fine striations or pore- 
canals are also seen traversing the entire thickness of the capsule. A single large oil- 
globule 1"75 mm. in diameter occurs in each ovum. This, as usual, constantly passes to 
the upper pole, just as the oil-globule does in pelagic eggs. Only a single unimpregnated 
egg was available for the demonstration of the early condition. In some unhealthy 
or dying eggs a number of very small oil-globules were seen clustering round the edge 

* M'Intosh, Nature, June 1886. 


of the germ, the general size being about 3^ inch. Towards the period of hatching 
the chitinous zona radiata is more easily torn, and readity splits into lamellae, all of 
which show minute punctures (PL XX. fig. 8), appearing like minute pale specks on 
a dark ground. In some again the punctures are lost in a general granular area. 
Whether these so-called punctures were actual canals, or only radiating striae, could not 
be demonstrated. 

This separability of the capsule into layers in the later stages does not conflict with 
the view that it is really a single coat. Such chitinous formations in other forms show 
the same tendency to split into filmy strata under certain circumstances, and, as explained, 
a like tendency is exhibited in the extremely thin zona radiata of Gadus morrhua 
and G. ceglefinus. In size the ovum of Anarrhichas* resembles that of the salmon 
(PI. XX. figs. 9, 10), though the punctures in the latter form (fig. 11) seem to be 
somewhat larger. 

Liparis montagui. — The capsule presents externally a minutely areolate appearance 
(PL I. fig. 4) due to slight elevations, resembling indeed the surface of grained morocco 
leather, the elevations having a more or less marked linear disposition. In newly 
deposited examples, or in ripe ovarian ova, the external configuration shows an almost 
regular hexagonal character (PL I. figs. 21, 22), the sutures being pale, while the central 
regions are more opaque, probably from increased thickness. After exposure to water a 
change seems to occur, the hexagonal facets becoming less marked, while a series of eleva- 
tions become visible, and are apparently due, therefore, to a later modification. In 
oblique views the capsule shows undulating surface-markings (PL I. fig. 22). As these 
ova were not actually observed to be deposited by this species, however, it must be added 
that a margin of doubt exists as to the feature described. 

In this as in other species the zona radiata is at first soft and pliant, hardening 
subsequently, as in those deposited in the Laboratory. In the fresh condition minute 
punctures are visible, though these are less distinctly seen after mounting in certain 
media, e.g., Farrant's solution, and on tearing the capsule the same dense series of laminae 
can be separated as in Anarrhichas and Cottus. While in the ovary the eggs have a pale 
straw colour, and measure about '043 inch, the oil-globule being *0083 or less, but those 
just deposited in the tanks show a slight increase in size, viz., "045 inch in diameter, 
and the oil-globule varies from *005 to '0116 of an inch. The eggs of this species are 
very frequent on sea-w T eeds, zoophytes, and fragments of sticks and debris at the bottom, in 
comparatively shallow water as well as in the deeper parts, and they show much variation 
in colour, from pale straw to a light pink or flesh colour. They have often been mistaken 
for the eggs of the herring, from which they differ in regard to the structure of the 
zona radiata, and in the absence of the so-called vitelline membrane, which Mr Brook, 
however, says is not present. The embryos again are sufficiently diagnostic, for the 

* It is remarkable that the masses of the eggs of this species have hitherto escaped observation, fishermen being so 
little acquainted with them that they were mistaken for those of the salmon. Some time afterwards the recently 
hatched embryos (then unknown) were recognised by one of us in Edinburgh, having been forwarded to the Fishery 
Board for Scotland by one of the steam trawlers of the General Fishing Company, Granton. 

VOL. XXXV. PART III. (NO. 19). 5 R 


elongated and somewhat feeble herring cannot be confounded with the shorter and more 
vividly tinted larval sucker, which shoots into the surrounding water at once on issuing 
from the egg. The ova usually referred to this species, however, require further study, 
and the condition of the larva on emergence presents certain differences in the several 
varieties. It is possible that several species have similar ova, but where absolute 
certainty in regard to their determination was not possible, only those having the same 
size and structure were grouped under the head of this species. The spawning period 
ranges apparently from January to June. 

Cyclopterus lumpus. — The ova of this species are very variable in colour, ranging 
from a beautiful amethystine lustre through the various shades of straw-tint to greenish. 
The zona radiata is thick, and minutely punctured, but presents no special thickenings 
or superficial wrinkles, except where the facets of attachment are situated. The eggs are 
fixed together in sponge-like masses, so as to permit free aeration, yet the hatching of 
this species in confined tanks is somewhat difficult. The germ, as in pelagic forms, keeps 
for the most part at the lower pole, the oil-globules ascending to the upper pole. Their 
diameter is about '1 inch, while that of the large oil-globules is about *041 and less. 

Ova, apparently of this species, were obtained in great quantity from the stomachs 
of codling off Boarhills (Fifeshire), but, unless erroneously diagnosed, the gastric juice 
had caused a diminution in diameter, as they measured only , 083 of an inch, while 
the large oil-globule in each measured # 026, and one or two smaller globules were also 
present. Though to a slight extent digested, this ovum showed much resemblance to 
that of Cyclopterus, and formed masses of a yellowish green colour. In addition to the 
ordinary punctate structure, the zona agrees with that of Cottus in presenting larger, 
more evident dots at intervals (PI. I. fig. 24) ; indeed, this arrangement of larger punc- 
tures in the midst of the smaller ones is more distinct than in Cottus. They resemble 
large canals rather than radial striae, and they are finely dotted when viewed in profile, as 
at the edge of a torn fragment. It is noteworthy that at the same period as the above 
partially digested ova were obtained, a considerable quantity of similar eggs of a pale 
straw colour were procured on the beach near the Laboratory. Their diameter was "0916 
and that of the large oil-globule '031, and several smaller globules were also present. 
The ordinary pores were larger than in Cottus, but the larger pores, scattered at intervals, 
were similar. If these be the ova of Cyclopterus, which they closely resemble, consider- 
able latitude must be given in regard to diameter. It has, however, to be borne in mind 
that the condition of the ova (i.e., whether they had been subjected to dessication or other- 
wise) was unknown. The spawning period of Cyclopterus extends from February to the 
end of May, and occasionally even a little later. 

Agonus cataphractus. — The ovaries of a number of female specimens caught by the 
sprat-nets in the estuary of the Tay were found to show nearly ripe eggs in December. 
The eggs are large, and of a dull golden or dull yellow colour, their diameter being "07 
inch and that of the oil-globule about "0216. The zona is minutely dotted with punctures 
arranged in a linear series. The surface is also covered with well-marked areolae. (PI. I. 


fig. 23). This species seems to spawn from January (or perhaps even from December) 
to April. 

Cottus scorpius (PL I. fig. 3). — The ova present various shades of red, inclining at 
times to orange or yellow. Their diameter averages "075 inch, and the large oil-globule 
ranges from "015 inch in diameter downward. The zona is smooth, except where the facets 
for attachment to adjacent ova occur. Minute dots are visible under a high power, and 
these have a more regular linear arrangement, as a rule, than in Cyclopterus. Moreover, 
larger dots occur at intervals all over the surface, recalling those noted in Cyclopterus 
taken from the stomach of young cod. In the Eeport to H.M. Trawling Commission,* 
one of us has alluded to the error of Professor Alexander Agassiz in considering the 
ova of Cottus pelagic, a fact overlooked by Mr Cunningham^ 

Ammodytes tobianus, L. — G. 0. Sars states that the comparatively large ova of this 
species are not pelagic, but are laid in loose sand, where they go through their 
development. Couch, again (No. 44, iii. p. 138), considered that it sheds its ova 
in this country as it dashes through the sand in December; while Day (No. 51, i. 
p. 333) found the reproductive organs in both male and female, at St Ives, far 
advanced in August and September. On the other hand, Thompson states that in 
Ireland they were nearly ripe at the end of July. The organs, however, were found 
to be small in November at St Andrews. Early in May some specimens (none more 
than 6 inches in length) showed ripe spermatozoa, though the testes were comparativelv 
small ; while in the females the ovaries were not much developed, and contained 
very minute eggs. These eggs were transparent and granular, with a large germinal 
vesicle. Some larger eggs, five or six times the diameter of the remainder, showed 
a coarsely granular yolk, with many small oil-globules, and a very thin external 
capsule, which is finely reticulated, and provided with minute punctures as in other forms. 
In the ovary the eggs appear to have a somewhat whorled arrangement. Later, about the 
beginning of June, the reproductive organs in about twenty examples showed an irregular 
state of advancement, some having fairly advanced ovaries, while others were rudimen- 
tary. In those best developed the ova were of a rich orange colour, "reddish yellow," as 
Sars said, and they were just visible to the naked eye as minute grains -^ of an inch in 
diameter. The germinal vesicle was still very evident, measuring ^^ inch. Most of 
the larger ova were of this size, though others were much smaller, the smallest being in 
fact less than the nucleus of the largest eggs, and their nuclei showed many nucleoli. The 
zona is distinctly dotted at this stage. The sperms in the male fishes showed a distinct 
head, but no motion was visible at this time. So far as could be observed at St Andrews, 
the spawning period of this species would seem to be late, indeed so late as to bring it 
within a reasonable distance of the pelagic larval forms described in a subsequent 
chapter.}: In some examples, however, examined in the middle of December, the genital 

* 1884. t Op. cit., p. 103. 

t Section xi. Investigations, at present being carried out by Mr W. L. Calderwood at the St Andrews Labora- 
tory, may clear up the subject. 


organs were so little developed as to form two rounded cords. From the fact that no 
definite series of pelagic ova has been found previous to the appearance of the larval 
forms, the ova would appear to be demersal. 

Gobius ruthensparri. — In a female specimen about 3 inches long, obtained on 25th 
January 1886, the ovaries were found to be small, though the ova were sufficiently 
developed to be visible to the naked eye. Under the microscope, ova of various stages 
were seen, the largest being about a line in diameter. A germinal vesicle was present, 
and the central region of the egg was filled with well-marked globules (yolk). 

Centronotus gunnellus. — Like Zoarces viviparus, this species is characterised by the 
presence of a single unpaired reproductive mass in the form of a median band between 
the intestine and the abdominal roof. Unlike Zoarces, however, the male organ of the 
gunnel is also unpaired. In Day's recent work on British Fishes the following note upon 
the spawning of this species occurs (No. 51, vol. i. p. 210) : — " Nilsson states that its 
spawn is deposited in November. Mr Peach, however, in June believed he discovered 
the spawn of this fish in Fowey, in Cornwall." At St Andrews, where it is abundant, 
frequent examination of the reproductive organs supports Nilsson's observations. In the 
earlier part of the year (February) the ovaries of the female are very slightly developed, 
minute ova at various stages occurring in the follicles. In May the male elements are 
less prominent than the female, for the ovary is the larger organ. In many the structure 
is in a state of degeneration, large fatty globules and other granules taking the place 
of the sexual elements. The ova at this time still show great variation in size, the 
germinal vesicle being also present and unaltered. Towards the end of November 
females, though of small size, present a large, clavate ovary, tapering from the liver in 
front to a point behind the anus. The ova are now readily seen by the naked eye, and 
those on the surface are of nearly uniform size, viz., about # 043 in diameter. Several 
oil-globules ("012 in diameter) occur in the larger ova, and the yolk is opaque on 
account of the abundant straw-coloured, almost opaline, yolk-spherules. Outside the 
ovary is a transparent membrane, apparently continuous with the interstitial connective 
tissue, and not readily removed from the surface. The smaller ova are finely granular, 
and in some (the larger) small oil-globules are present. 

The ova of this species after deposition seem to have been first observed by Mr 
Anderson Smith,* a member of the Fishery Board for Scotland. He found them, 
probably on the west coast, from February to April. At St Andrews they have hitherto 
been obtained amongst the rocks in March, masses about the size of a walnut (as Mr Smith 
states) occurring in the holes of Pholas, the adults in each case being coiled beside 
them. The ova adhere together like those of Cottus or Clupea harengus, and have 
a diameter of '076 inch, while the oil-globule measures "0166 to "016 inch. In 
those nearly ready to hatch the zona radiata is somewhat tender, and presents the 
usual laminated appearance. It is also most finely and regularly punctate, after the 
fashion of wire-cloth of great tenuity. The ragged margins especially exhibit the 

* Quoted by Mr Cunningham, op. tit., p. 125. 


appearance of finely crossed fibres, partly due in all probability to the breaking up of 
the tissue. 

Blennius pholis. — In May a large male, 6f inches long, was procured at the East 
Rocks, St Andrews. The testes were highly developed, and almost reptilian or 
amphibian in appearance. They form two large flattened organs, or rather are rounded 
anteriorly, and flattened on the inner side — the two bodies, in fact, being precisely 
like the two separated halves of a long bean. The blood-vessels run along the flat sur- 
face, and give off branches which spring as it were from a midrib. In colour they are of 
a faint pinkish white. The outer or convex region is of a firmer texture and more 
translucent than other parts of the testis, being composed apparently of tubules contain- 
ing spermatozoa in full activity and abundant sperm-cells. The whiter opaque region 
consists of aggregated sperm-sacs. The spermatic duct leading to the genital aperture 
is exceedingly wide, and on one side shows a spermathecal enlargement, which, at 
first sight, resembles an additional urinary bladder. The ducts open by an aperture 
on a prominent papilla behind the large corrugated anal orifice. This strong papilli- 
form protuberance approaches that in fishes which are known to copulate, but there 
is no account of such in this species. A little later (viz., on the 23rd June) an adult 
female, 5 inches long, had the ovaries much enlarged — containing a mass of large 
bluish-grey ova, and smaller ones of a slightly orange hue. The minute structure of these 
somewhat peculiar ova has been carefully described by Dr Scharff.* The ova (which 
were not quite mature) measured about "0415 of an inch in diameter. 

The above facts show that this species deposits its eggs apparently during the early 
summer ; Parnell, indeed, names the month of June, while Dunn considers that it 
spawns in spring. Couch states that it deposits the ova on the roof of small caverns in 
rocks near shore (Zool., 1846, p. 1419); and Day, who quotes the above authors, adds 
that he found minute fry at Penzance in August. At St Andrews young specimens, 
about an inch long, and which had acquired the features of the adult, are abundant 
in the pools of the East Rocks about the middle of September. 

Blenniops ascanii. — On 14th June 1886 a fine male, procured in a crab-pot off the 
Buddo Rock, Fife, showed testes only partially developed. The stomach was distended 
by eggs of Cyclopterus, upon which it had been feeding largely. 

A female in August exhibited only traces of ova — the ovaries being apparently 
atrophied, but on the 16th September both organs were very large, the individual ova 
reaching about ^ inch in diameter. 

Motella mustela. — On 17th July 1885, a female rockling, 6 inches long, was 
examined, and the ovaries were found to be connate posteriorly, and contained ova of 
some size, so that the species must pair very early in winter, and the spawning period 
would seem to be very lengthened. In May the tanks in the Laboratory were found to 
be full of the floating ova of this species, and during March, April, and May the ripe eggs 
appear usually to be ready for extrusion, so that the ova of the female above referred 

* Proc. Roy. Soc, vol. xli. (1886) p. 449 ; and Quart. Jour. Micr. Sci, Aug. 1887. 


to, in which the unripe ova were of variable size, large and small, would probably have 
been retained until the end of winter or beginning of the year. The pelagic ova of this 
species are amongst the most abundant forms in and beyond the bay in March, April, 
and May. 

We have already spoken of the capsule as a zona radiata — a protective membrane of 
general occurrence in the ova of most diverse groups of Vertebrates. Thus in the Aves 
a zona radiata is present, though it does not persist ; but at an early stage it dis- 
appears, and the egg leaves the ovarian follicle enveloped by another membrane which 
is distinguished as the vitelline membrane. This second membrane is exceedingly 
attenuate, so that it is difficult to distinguish it from the outermost layer of yolk- cells 
from which it is derived. The Reptilia possess also two membranes ; but, unlike the 
birds, they are not both of vitelline origin, the outer, which is very thin, Eimer (No. 53, 
p. 418) declares to be a product of the follicular epithelium, and therefore chorionic ; but 
the inner is thicker, and vitelline in origin ; and Eimer regards this as the zona radiata 
(his zona pellucida). The capsule in the Amphibia (Rana) is a remarkable structure, 
and would appear to be really a chorio-vitelline membrane, for the inner cells of the 
ovarian follicle form a layer very closely applied to the true vitelline membrane, 
and as the latter becomes continuously thinner the two layers are really inseparable, 
and form one layer, of which the outer stratum is chorionic, and the inner one is 

In the Elasmobranchs a double layer is present, the outer being first formed, and 
regarded by Balfour as vitelline ; while the inner one, equally of vitelline origin, he 
distinguished as the zona radiata. Both, however, atrophy as a rule before the egg 
leaves the follicle. In Cyclostomes (Petromyzon) two layers are described, an outer 
imperforate, and an inner perforated layer. The outer layer on contact with water 
swells up and forms a gelatinous coating by which the eggs adhere to external objects. 
In Myxine, according to J. T. Cunningham's researches, the thick capsule is a chorion, 
being developed along with its solid projecting processes from the follicular cells. 
Possibly a delicate vitelline membrane may be developed internal to the outer homo- 
geneous capsule, but this Mr Cunningham was not able to decide (No. 46, p. 600). 
Notwithstanding that a double egg-membrane is so common, as indeed Kolliker long ago 
pointed out (No. 80, p. 84), yet in the Teleostei the recognition of a single layer of vitel- 
line origin accords best with the character of the capsule in general, in the mature ovum.* 
Certainly Lereboullet's designation "chorion" (No. 93, p. 459) is inappropriate ;t nor 
does Kupffer's view, that the capsule in certain osseous fishes is double, like the 
Elasmobranchs, seem better justified. Kupffer holds that, in the case of Clupea 

* Dr Martin Barry affirmed that one membrane only envelops the ovum in fishes, no layer being formed 
external to the vitelline membrane (No. 21, p. 309). Solger also came to the same conclusion from an examination of 
Leuciscus rutilvs (Arch. f. Mik. Anat., 1885). 

t Lerebodllet, however, later in the same treatise, refers to the chorion as of vitelline origin, and "produced by 
the primitive vitelline membrane separated from the vitellus " (No. 93, p. 507). 


harengus, two separate layers, an outer vitelline membrane, and an inner zona radiata, 
maybe distinguished (No. 87, p. 178); but Hoffman does not think the distinction 
justifiable — one membrane alone being present, which, however, presents an inner less- 
defined part, probably more recently formed, or in course of formation, from the vitelline 
cortex; and G. Brook supports this interpretation (No. 34a, p. 201).* If the outer con- 
centrically-laminated stratum be regarded as a layer separate from the inner stratum 
which shows radial striations, then with Kupffer we must consider the former as of 
exceptional occurrence amongst Teleosteans (No. 87, p. 178). Brock, again, figures two 
Teleostean ova with double capsules, the outer layer being striated in one case and 
unstriated in the other (No. 29, Taf. xxviii. fig. 7, f. ; Taf. xxix. fig. 6, b, e). The 
interpretation as a single layer, we repeat, seems, however, better founded, for if the ovum 
of Callionymus lyra be examined, we find external to the zona radiata, which has the 
usual structure, "a series, for the most part, of hexagonal reticulations like those of a 
honeycomb," not unlike the reticulation of the early ovum of Ammodytes tobianus. 
" These spaces are not quite uniform in size, but many are. Some again have four, six, 
or seven sides ; . . . . the septa bounding the reticulations stand out very distinctly, and 
their edges show minute striae " (No. 106, p. 481, also PL xiii. figs. 1, 2, 3, 4). The ripe ova 
of this species have been examined at the Marine Laboratory, and the reticulation in 
both cases is external, and is evidently inseparable from the zona radiata. The same 
condition would appear to be present in the pelagic ova of Crenilabrus tinea, recently 
described by J. H. List, the outer part of the zona consisting of regular six-sided 
areas, and the inner only of fine parallel striations.t Such elaborate modifications 
of this single layer are probably illustrated by the ovum of Cyprinus dolbula, with 
its radially directed rod-like processes; of Perca jiuviatilis (No. Ill, p. 186), with 
its prominent hollow cylindrical appendages, which interlace, and, with the mucous 
envelope, hold the eggs together in " elegants reseaux," as Lereboullet describes 
(No. 93, p. 471); but they do not serve, as the same author states, for absorption 
like the minute canals, though both structures penetrate the capsule. In such forms also as 
Blennius, Gobius, and pelagic eggs like Heliasis and Belone, long filaments occur near the 
micropyle, and are pronounced by Hoffman, who describes them, as simply excrescences 
of the zona radiata. If we regard the capsule in Teleosteans as essentially a single layer, 
then the dissimilarity of the elaborately modified capsules of the foregoing species 
— of the less complex capsule in Clupea harengus (No. 87, p. 178), and in JEsox lucius 
(No. 93, p. 465); and of the extremely simple membrane in the ova of Gadoids, Pleuro- 
nectidse, and others, wholly disappears. The species in which various layers, not to 
say distinct membranes, have been described, find their place in the same category as the 
ova of the cod and like forms with simple layers. That the capsule can undergo elaborate 
modification is easily understood, when it is noted that in its early condition it is always 

* See also Lereboullet's description of a similar inner layer closely applied to the yolk in the pike, the outer 
stratum being alone striated (No. 93, p. 465). 

t Zeitsch.f. wiss. Zool., Bd. xlv. (1887) p. 596, tig. 1, a, b. 


soft and pliant, and may remain so even after deposition, as we find to be the case, 
notably in the thick capsules of Gastrosteus spinachia and Cottus. These ova, for some 
time after deposition, are soft and yielding, possessing, as Prof. Allen Thomson (No. 153) 
states, in the fresh-water congener of the former, " so little elasticity that it usually 
retains dimples or impressions made upon it from without." In this connection it may 
be mentioned that the so-called outer laj^er in Clupea harengus is slightly facetted (No. 
87, p. 177), this being due, doubtless, to the impress of the follicle-cells before the egg is 
extruded — a suggestion which may also be applied to the similar appearance in the case 
of Perca (No. Ill, p. 187). The zona radiata, as its name implies, has a characteristic 
radiate structure in many Teleosteans. The real nature of the striation so visible in section 
has been much disputed, and there is little unanimity of opinion in regard to it. In 
many species this feature has not yet been made out, e.g., in a number of familiar 
Gadoids, viz., G. merlangus, G. ceglejinus, G. luscus, Molva vulgaris, and some of the 
Pleuronectidse, such as P. fiesus and P. limanda. The capsule in the familiar 
Pleuronectid, Pleuronectes platessa, again, is very distinctly punctured (PI. I. fig. 20). 
Cunningham has recently mentioned that the zona radiata of the cod usually described as 
not punctured (vide Ryder. No. 141, p. 457), exhibits pore-canals, but he does not describe 
them in the ovum of Trigla gurnardus ; yet the latter, so far as our experience goes, 
shows them much more distinctly than those of the cod ; indeed, we have not yet 
satisfied ourselves concerning the latter. In the ovum of Trigla one of us has demon- 
strated that the whole surface of the capsule is minutely and faintly dotted (PI. I. fig. 
19). This punctate appearance is especially distinct after the escape of the embryo. 
The capsule of this form in the unimpregnated condition shows numerous wrinkles — the 
yolk occupying a comparatively small area, so that a large perivitelline space exists, 
which, however, diminishes after fertilisation, until the vitelline globe almost fills the 
capsule, which at the same time becomes less distinctly wrinkled. The corrugation of the 
zona radiata is, however, a characteristic feature, and exists in all the eggs of this species. 
The zona is firm and elastic to a remarkable degree for a pelagic form, and its unevenness 
causes some obscurity — only a faint line of dots being as a rule visible along the ridge 
which happens to come into focus under the microscope. In one instance the zona 
presented a series of scale-like markings or areolae (PI. I. fig. 16), probably due to an 
unusual or morbid condition in connection with the follicular epithelium. The normal 
wrinkles (seen best in T. gurnardus) also occur in the lemon-dab (PI. I. fig. 18); and 
Ryder speaks of these in G. morrhua as fine lines crossing each other at definite angles. 
Such lines, however, are less visible in eggs which are healthy and perfectly mature. 
The typical zona radiata exhibits, as Von Baer discovered in Cyprinoids, fine striations 
perpendicular to the superficies of the yolk, and Carl Vogt described at greater length 
the same feature in the Salmonidse (No. 155, p. 7) ; while Reichert noted it in the ova 
of Tinea vxdgaris and Leuciscus erytlirophtlialmus, and Leydig in Gobius jluviatilis. 

Are these strise really canals, or merely fine fibrillations, such as we find in the 
transient zona radiata of the fowl under a high power ? In either case a punctured or 


dotted appearance would be produced superficially, as in a large number of Teleostean 
eggs, especially in the comparatively dense capsules of fresh-water forms. These 
punctures may be comparatively large and distinct, as described by Muller in Perca 
(No. 1 1 1, p. 1 87-8), and by Leuckart in Esox; or they may be of smaller size, as in Salmo 
fario (No. 153, p. 101, and fig. 68, c, d), Gastrosteus spinachia; or of extremely minute 
size, as in certain pelagic forms, e.g., Trigla gurnardus and P. platessa. Frequently 
the striations are observed to pass only partially through the capsule, and the outer 
stratum is then imperforate, instances of this condition being the capsule of Clupea 
(No. 87, p. 177), and Esox (Aubert), Gastrosteus spinachia, and probably Trigla 
gurnardus. In other ova they traverse the whole thickness of the capsule, as is the case 
in Salmo fario (No. 4, p. 198), and in Perca fiuviatilis, according to the experiments 
of J. Muller (No. Ill, p. 188). The distinguished observer just named was convinced 
that, when he placed the eggs of the perch under pressure, oily matter from the interior 
of the egg could be squeezed through the canals of the zona radiata, and the canalicular 
structure of this membrane would appear to be demonstrated in this instance. Other 
observers, however, strenuously deny this, and, like Andre, pronounce the so-called 
canals to be nothing more than rectilinear striations directed radially from the inner 
to the outer surface of the capsule (No. 4, p. 202), precisely like the radial fibrillations 
in the zona of the fowl's ovum. It is only necessary to observe the effect of desiccation 
on the egg of the cod, and then the action of water, to prove that a ready interchange 
occurs through the zona either by pores or by ordinary endosmose. 

Little can be said here as to the origin and growth of the zona radiata, for its 
development is already complete when the ovum reaches maturity. That it is a true 
vitelline membrane admits of little doubt ; and Hoffman's opinion, that it is secreted 
by the vitelline mass as a superficial layer during the intra-ovarian period, and is not 
separated until it shows an appreciable density and firmness, is probably well founded.* 
Eansom holds that, after it is defined as an external membrane, it continues to grow 
interstitially up to a certain stage, when growth ceases, and it performs a jsolely 
protective function (No. 127, p. 494). Other layers are formed later upon the surface 
of the yolk after the zona radiata has become detached as an elastic protective capsule, 
and these may claim to be called vitelline membranes, as indeed they have been styled 
by various authors. Thus, Oellacher, when speaking of the vitelline membrane in the 
trout (No. 113), really means the stratum of germinal protoplasm, the polar segregation of 
which forms the blastodisc ; while Lereboullet uses the same term for the layer 
of protoplasm which ventrally limits the intestinal tract of the embryonic fish at a 
comparatively late stage (No. 93, p. 612). Such uses of the term " vitelline membrane" 
for very different layers (though all of vitelline origin) are not to be approved, and the 
name zona radiata is at once distinctive and appropriate for that vitelline membrane 

* The development of the vitelline membrane in Triton has been shown in an interesting manner by Mr Iwakawa, 
and his descriptions and admirable figures (see No. 75, p. 274, and pi. xxiv. figs. 24-26) will apply in the case of the 
Teleostean capsule. 

VOL. XXXV. PART III. (NO. 19). 5 S 


which forms the external capsule, and subserves a protective function. When the 
embryo is sufficiently mature the capsule is burst, — the rupture being due, no doubt, 
to the vigorous motions of the young fish, which in the case of Pleuronectes fiesus 
generally emerges from the capsule by pushing out its tail. 

The Micropyle. — The zona radiata is pierced by the micropyle (PL II. fig. 19, raic.), 
an aperture probably universally present in Teleostean eggs, and in these it varies 
very little in structure and appearance. Thus in the salmon, trout, pike, ruff, perch, 
bullhead, gudgeon, minnow, chub, and various species of Gastrosteus, Eansom's descrip- 
tion accords almost perfectly with the micropyle, as seen in the cod, haddock, ling, 
whiting, bib, flounder, dab, plaice, gurnard, and others. At a certain point the capsule 
is distinctly thickened, and an internal conical elevation is formed, which, as Balfour 
says (No. 10, p. 51), corresponds with an external funnel-like depression, while a 
cylindrical canal connects the bottom of the funnel with the apex of the inner papilla.* 
The thickened appearance of the capsule in the micropylar region is not produced simply 
by the protruding hillock, and due to the crateriform depression outside ; but as Andre 
(No. 4, p. 201) ascertained, and as may be easily demonstrated in the delicate translucent 
ova of the Gadidse or Pleuronectidse, the capsule is actually thicker at this point 
(PI. X. fig. 9). List shows the same feature in Crenilabrus tinea} Viewed from 
above, three parts may be distinguished — a large outer annulus and a smaller inner 
ring, with a central pore which is the opening of a cylindrical tube. In the trout 
these measure, according to Andre, *015 mm., '008 mm., and - 005 mm. in diameter 
respectively. The first-named ring is the rim of the external crater; the inner ring 
marks the narrower, deeper portion ; while the central aperture is the essential part, the 
true microlpyar canal, which is not, however, perfectly cylindrical, but midway along its 
course distinctly enlarges, and then narrows again. This sinuosity observable in the canal 
proper, Andre thinks, is produced by the ends of the pore-canals or radial striae which jut 
out slightly into the lumen of the micropyle (No. 4, p. 201). That the micropyle is really 
a depression, and not simply a puncture, is shown by the fact that the striations of the zona 
radiata present an inclination towards the micropyle, which is increased as the aperture is 
approached, and still more so down the walls of the crater, their outer ends being directed 
towards the cavity of the depression, and forming projections into it as just stated. 

This inclination of the striae is shown by Ransom and others ; but His does not 
indicate it in his figures of the ovum of Salmo fario and S. salar (vide No. 67, Taf. i. 
figs. 7, 8, 9, and 10) ; and the same may be said of List in his recent paper. Connected 
with the depression and thickening of the capsule around the micropyle, is the striking 
appearance external to the larger annulus seen in the marine and fresh- water species of 
Gastrosteus, where bold radiating striae pass away from the margin of the external crater 
(vide Nos. 1 53 and 67, Taf. i. fig. 15), a feature less marked in the chub, in which the margin 

* Ransom speaks in Gastrosteus of the micropyle as projecting actually into the protoplasmic disc, and of a subse- 
quent shortening of its funnel after fertilisation (No. 127, p. 450). 
t Op. cit., p. 597, fig. 2, a. 


is crenate and the sides are furrowed. In the pike Ransom describes the micropyle as 
trumpet-shaped, and projecting slightly from the surface of the capsule (No. 127, pi. xvi. 
fig. 25, a) ; while in the minnow, too, the margin is raised around the outer opening of 
the funnel (Ibid., p. 456). Striae are occasionally seen in certain pelagic forms, e.g., in 
Trigla gurnardus and Gadus ceglefinus, but the margin of the crater is usually sharply 
marked, and the aperture itself very clearly defined without radial markings. When viewed 
in " full face," the funnel seems larger than it really is on account of the torsion, so to 
speak, of the zona radiata, which appears as if bent in to form the orifice, a feature Andre 
particularly points out (No. 4, p. 199), and to which we have made reference above. 

The micropyle thus varies in appearance. Usually the external opening is the larger ; 
but in some cases this is reversed, a large gaping internal opening being present (vide 
fig. of ovum of G. ceglefinus, PI. I. fig. 14), while the external orifice is small. The 
striations above mentioned are also visible in this case — the whole peri-micropylar region 
being granular, while the granules have a tendency to range themselves in radial lines. 
Near the micropyle in some examples an accessory structure is present, due apparently 
to a granular protrusion of the zona (PL I. figs. 11 and 15). In this and other cases 
the micropyle was distant from the germinal area. Fertilisation in pelagic eggs does not 
produce any marked change in the micropyle, certainly none like that described by 
Ransom, and just mentioned. In one instance, beside the micropyle proper, was a 
depression plugged by an ovoid granular structure, while a large group of " oleaginous " 
spheres lay upon the yolk near the micropyle (PL I. fig. 17). 

Origin, Position, and Function of the Micropyle. — The mode of origin of the 
Teleostean micropyle is unknown. When first observed in the mature ovum it presents 
the features maintained throughout the subsequent history of the egg. Leydig describes 
(No. 97, p. 376, fig. 6) the earliest ovarian egg of Trigla hirundo as somewhat pyriform 
and stipitate, recalling, in fact, the stalked ovum of Unio, in which the micropyle marks 
the pedicular point of attachment by which the egg adheres to the ovarian capsule, as 
Carus was the first to note. Such an interpretation of the micropyle, as the cicatricule 
left by a pedicle, cannot in the case of the osseous fishes be adopted, and we are still 
left in doubt as to the way in which the aperture arises. 

It is interesting to observe that in many forms the position of the micropyle is 
constant, and corresponds to the germinal pole, where the embryonic area is formed, 
as, indeed, Ransom found in Gastrosteus. In Perca, however, the aperture is turned 
towards the inside of the egg-tube — the ova being fixed in a cylindrical mass, so that the 
possibility of the micropyle being blocked up by adjacent ova is obviated (No. 127, p. 456). 
Gerbe similarly says, " that the micropyle plays an important part, as the disc always 
collects near the place occupied by it" (No. 57, p. 330). Neither Ransom nor Gerbe 
examined pelagic ova ; but from the later observations of Ewart and Brook, it would 
appear that in floating eggs the micropyle is always found in the lower hemisphere 
(No. 55, p. 55). This position is, of course, the reverse of that in stationary demersal ova, 
in which a preformed disc is commonly found in the upper (animal) segment ; whereas in 


pelagic eggs the blastodisc formed after fertilisation is also theoretically constant, but in 
the reverse segment — the animal pole being underneath, and in calm water the germ 
is usually found at this lower pole. # 

As to the function of the micropyle, most authorities are agreed that it is connected 
with the fertilisation of the ovum, affording means, in fact, for the entrance of the 
spermatozoa. Kupffer, however, calls this generally adopted view into question, and 
doubts whether it has any essential part to play in fecundation (No. 87, p. 179). In the 
ova of lower forms the function named has been universally admitted from the time 
Meissner first described the aperture in crustaceans and insects (No. 102, p. 272), 
and Leuckart laboriously worked at the structure and function of this aperture in a 
large variety of insects. The latter, in his elaborate paper, states that he beheld sperms 
not only adhering to the outside of the egg, but entering the micropyle ; and indeed figures 
this phenomenon in the ovum of Melophagus ovinus, a crowd of spermatozoa being 
collected at the external opening, though not more than three or four find entrance. 
In Teleosteans its function appears to be solely that of affording ingress for the fertilising 
element, though Ferd. Keber (No. 77) conceives not only this to be the case, but that 
through it there is an actual outflow of the contents of the egg — the purpose of this 
outflow being to lubricate the canal and favour the entrance of sperms, as well as to 
increase the vacant space within for the reception of the spermatozoa. 

Meissner, who first described the micropyle in the ovum of the rabbit, thought that 
the aperture only penetrated the vitelline membrane, and that it was effectually closed 
over by the chorion outside (No. 103). A modified view has been put forward by 
Ransom, who was probably the earliest to discern and rightly interpret this aperture in 
osseous fishes.t He was of opinion that a delicate film covered the micropyle, which 
was only ruptured by the entrance of sperms ; and more recently Boeck, in connection 
with his remarkable theory of osmotic fertilisation, to which we shall refer shortly, 
conjectures that a clear membrane, in the case of Clupea harengus, closes the aperture of 
the micropyle (No. 23, pp. 5, 6). Besides admitting sperms, a small quantity of water- 
may also enter, which (water) mingles with certain organic particles, and fills up the space 
between the vitellus and the zona radiata in the extruded ovum. 

The Deutoplasm or Food-Yolk. 

Within the egg-capsule is the ovum proper, a spherical translucent mass, largely 
composed of fluid food-yolk. With the food-yolk, which serves for nutrition, there is 
interfused active protoplasm, and this, at an early stage, collects as a delicate film over 
the surface of the yolk-ball ; indeed the mature ovum of Teleosteans, before fertilisation, 
exhibits a distinct superficial layer of clear protoplasm, in which minute vesicles and oil- 

* According to Ryder, the germ is lateral in A.losa. 

t Bkuch independently discovered the micropyle in the eggs of the trout and salmon (No. 35, p. 172). 


spheres are embedded.* During the first hour after fertilisation these translucent vesicles 
are readily seen under a moderate power (450). Occasionally granular protoplasm is 
observed at certain parts of the contour of the vitellus in the haddock. A similar appear- 
ance occurs in the cod, in ova which are abnormal though still translucent. Amongst 
these vesicles are others extremely minute and very numerous, which in refracted light 
have the appearance of punctures. 

To distinguish the albuminoid matrix, which forms the greater part of the bulk of the 
egg, from the active germinal protoplasm, the name " deutoplasm," conferred by Prof. 
E. van Beneden, is both appropriate and convenient. This deutoplasm rarely has the 
appearance of yolk-segments contained in a sponge-like network ; but is composed in 
many pelagic eggs of minute yolk-particles aggregated in a matrix apparently homo- 
geneous, highly refractive, and coagulating on the addition of water. The latter feature 
has long been known, for Lereboullet found coagulation to take place in the ovum of 
Salmo fa rio, just as Vogt had noted in Coregonus palcea (No. 155, p. ll).t Broadly 
speaking, we may say of the yolk in the Teleostean ovum that it possesses special features 
of its own, which separate it from the nutritive matter of other vertebrates ; whereas the 
yolk of the Elasmobranchii resembles in a very marked manner that in the Avian egg. 
There is apparently little difference in the specific gravity in various parts of the deuto- 
plasmic matrix, as it retains any position in which it is placed before the aggregation of a 
polar disc ; but Ransom questions whether its specific gravity is equal throughout, and 
thought that nearer the surface it is of a more fluid consistency, or, as he says, " I had 
some reason to think a little less dense than the centre, as it ran more freely ; but all parts 
flowed from a rupture like very thick syrup " (No. 127, p. 436). The greater density of the 
deeper deutoplasm can be readily explained by the movement of the interfused protoplasm 
surface-wards, so that the central part of the yolk-globe becomes more purely yolk- 
matter, while with the more superficial strata a larger, though constantly diminishing, 
quantity of germinal protoplasm will still be intermingled. Kowalewsky considers that a 
protoplasmic network must exist in the yolk (Carassius, Poly acanthus, and Gobius), since 
after hardening the latter presents polygonal partitions (Zeitsch.f w. ZooL, vol. xliii., 1886). 
He also terms the yolk the entoblast, in contradistinction to the germinal disc or ectoblast. 

The freedom with which the so-called oil-globules in various forms (e.g., the gurnard 
and ling) move through the deutoplasmic globe not only proves its very fluid consistency, 
probably corresponding with that of thick cream, but shows the absence of a definite 

* Lereboullet inclines to the opinion that the yolk is active in the formation of germinal protoplasm ; " at any 
rate," he says, "in the Lizard and Bird we find it before the germinal vesicle is ruptured " (No. 95, p. 11). 

t The behaviour of the deutoplasm under various conditions was made the subject of some interesting observations 
by Dr Davy in the ovum of the charr (Salmo umbla), and he found that while contact with water in quantity coagu- 
lated it, the careful application of water in minute portions did not do so. Again, when heated even so high as 212° F., 
it did not coagulate, nor did it under the influence of steam ; whereas boiling water at once effected the change, owing, 
it was inferred, not to the heat, but to the admixture of water. While acids, salts, and alkalies had no coagulating 
influence, except when dilute, nitric acid, corrosive sublimate, and alcohol produced the change immediately. Davy 
came to the conclusion, as a result of his researches, that the deutoplasm of the charr and other Salmonoids has pro- 
perties distinct from the albumen of the Avian yolk (No. 50, p. 436). Results similar to those of Davy were obtained 
by Ransom in the various Teleostean eggs which he studied (No. 127, p. 436). 


protoplasmic network, such as the reptilian ovum presents, or as Dr Schultz demonstrated 
(No. 144) in the Selachians. 

Little or no food-yolk makes its way into the germinal area, so that, as Lereboullet 
observes (No. 93, p. 485), it takes no part in the segmentation of the germ. Indeed, all 
evidence tends to prove that the deutoplasm is in an inert or quiescent state, and only 
passively contributes to embryonic development, being slowly incorporated by the active 
protoplasm of the blastodisc in a mode which Ryder compares to the process of ingestion 
and assimilation in Amoeba (No. 141, p. 557). 

When the eggs of Gadus morrhua are partially dried, the surface of the yolk shows 
a series of clear reticulations, which on re-immersion in water run together and disappear 
in the course of eight or ten minutes ; such reticulations have, however, no connection 
with the later protoplasmic reticulation of the vitellus after epibolic extension of the 
blastoderm, and which is very noticeable in the cod, common dab (PL V. figs. 3, 11), 
and others. Haeckel regards it as so much passive matter contained in a gastrula- 
cavity (No. 62) ; but in Teleosteans it plays a more important role in later stages than 
that of supplying crude pabulum to the germ. Indeed, the germinal protoplasm 
Balbiani holds to be solely transformed yolk — not a mere segregation of interfused 
germinal matter. The germ, he says, is formed "by endogenous development of cells at 
the expense of the yolk or primordial protoplasm;" but he repeats the error of Coste 
that the germinal area is never formed until after fecundation (No. 9). 

J. T. Cunningham, in a highly suggestive paper, observes that the yolk and germ are 
equally concerned in the processes of cleavage ; segmentation in Teleosteans (as in 
Amphibians), dividing the ovum at the first stage of cleavage not equatorially, as E. 
van Beneden holds (No. 25), but meridionally into two similar halves, each with a cap of 
protoplasm and a mass of subjacent food-matter (No. 48). This view, however, gives 
to the crude deutoplasm an importance which cannot be accorded to it, even though 
cleavage as regards the yolk be merely potential and never fully achieved. 

The separation of the deutoplasmic mass into a segmenting blastoderm (PL XXII. 
fig. 1, bd), and an appended ball of pabulum (Ibid., y), is more complete in osseous fishes 
than in Elasmobranchs, and imparts to the yolk rather an accessory character than that 
of an active participator in the whole process of cleavage. 

That it contributes to the growing organism, and even buds off cells to build up part 
of the enteric tract, does not conflict with this view, which is supported by the fact that 
the yolk persists as a bulky appendage on the ventral surface of the young fish (PL XIX. 
figs. 5, 7), until a late embryonic stage, being enclosed by the body- wall, and finally 
absorbed when the post-larval stage is reached. The passive role attributed to the yolk 
Ryder would confine to the early stages, while later its function, he holds, is more 
important, since it becomes through the medium of the intermediary layer an active part 
of the ovum (No. 141, p. 569). 

But this view is not inconsistent with that here maintained, for if it serves as pabulum, 
this is really a part secondary to actual participation in blastodermic cleavage, and while 


the transformed substance of the cortex is difficult, if not impossible, to separate from the 
protoplasm of the germ proper, yet the yolk in the main is neither concerned in the 
cleavage of the germ proper, nor actively contributes to the increase of the embryonic 
tissues. In the Gadoids and other forms no vitelline circulation is established, and the 
absorption of the yolk is a slow and circuitous process.* 

Oleaginous and other Globules. — One of us has already published an account of 
these bodies, which form a striking feature in the yolk of many Teleostean ova 
(No. 125). The following remarks refer mainly to T. gurnardus, and they still further 
explain certain statements in the paper referred to. 

In the ripe ovum of the gurnard the globule (PI. V. fig. 5, og) is of a dull pinkish 
hue under a lens, while by transmitted light it exhibits a brownish yellow or pale salmon- 
tint. It is enveloped by a protoplasmic pellicle which sometimes appears incomplete, 
and forms an equatorial line, with pale pinkish vesicles studded along its border. 
Unlike those forms in which the globule is imbedded in a definite pocket {e.g., Motella), 
the globule in the gurnard, as also in Cyclopterus and Cottus (PI. I. figs. 2, 3), is most 
mobile, and can be made to pass under the disc when the latter is uppermost. On 
rolling the egg the globule emerges from beneath the disc, and is liberated with a bound 
at the edge of the rim. Moreover, in passing round it flattens out, and again contracts 
its diameter, or rather resumes its more spherical form. At times the globule appears to 
ascend directly through the yolk, though this may be a deceptive appearance, for 
Ransom found in the very mobile globules of Gastrosteus, that while the}' passed freely 
through the yolk, they could not be made to go "through its centre to get to the 
uppermost segments when the egg is rolled round ; in doing so the drops often separate 
to unite again" (No. 127, p. 436). Ransom accounted for this by the greater density of the 
central yolk-substance. The passage of the globule under the rim is well seen in the egg 
of the gurnard when the germ has extended as far as the equator. In certain morbid con- 
ditions the exact relations of the globule during its movements can be readily determined. 
Thus the globule is often firmly fixed in the dead egg between the opaque blastoderm and 
the yolk, or the globule is seen at the side, and cannot be made to pass beneath the disc, 
possibly on account of the doubling of the edge of the disc, or because the investment of 
the globule and the periblast have become dense and rigid. During the earlier morbid 
stages, however, the globule is observed to pass beneath the somewhat opaque disc, and in 
certain abnormal cases, from pathological change, the globule rolls external to the disc. 

From the above observations it is evident that Mr Cunningham's view (No. 48, p. 6; 
also PI. II. fig. 19) that the globule moves in the perivitelline space — that is, between the 
yolk and the zona radiata — is not borne out, since in experiments, such as the above, it 
passes between the disc and the yolk, and never passes through the protoplasmic cortex 
of the latter, save in rare morbid examples. In those eggs in which the rim has still a 
short distance to traverse the globule continues freely movable, and its surface next 
the yolk often presents a series of small globules and a single large central one. The 
* Vide " Significance of the Yolk in the Eggs of Osseous Fishes," E. E. Prince, Ann. Nat. Hist, July 1887. 


globule passes in later stages under the embryo, and for some time moves freely; but 
about the fifth day, when the blastopore closes, it becomes fixed, generally at the point 
coinciding with the vegetal pole. It now exhibits a thick layer of protoplasm, which 
becomes vacuolated in a complex manner, and gives origin to numerous nuclear structures 
as well as pigment-spots (PL XI. figs. 12, 13). In certain cases (gurnard) the peri- 
blast was observed to bend in from the blastodermic layers, and carry the oil-globule 
with it at its margin.* 

Perivitelline Space. 

This space is generally very distinct, and contains a transparent fluid, usually said to be 
water, which enters the ovum after fertilisation. In an undetermined species the space 
is enormously enlarged (PI. XIII. fig. 3). Eeichert, however, very lately observed that 
under the action of nitric acid it exhibited whitish flakes (No. 134, p. 463). Ransom again 
states that when the funnel of the micropyle is withdrawn from the discus proligerus (in 
Gastrosteus), water enters " to fill the breathing chamber." This view was questioned by 
one of us in a short paper read at the British Association in 1885 (No. 122), the following 
statement being made : — " That a certain amount of water finds access to this space is 
possible, but in stained sections the fluid filling the chamber often appears coagulated and 
faintly stained, thus indicating the presence of minute protoplasmic particles. It would 
appear to be, therefore, a dilute plasma." Raffaele has recently stated that the fluid 
is albuminous (No. 125a). In the gurnard various granular bodies, probably portions of 
protoplasm of a circular form, have been seen. It is possible that these agree with the 
segmenting corpuscles of Ryder and the expulsive bodies of Raffaele. t 

III. Extrusion and Deposition. 

The ova when ripe either pass directly into the body-cavity from the ovaries and out 
by an external pore, as in the Salmonidse and Anguilla, or they pass to the posterior end 
of the ovary as they become mature, and thence by an oviducal aperture to the exterior. 
The latter is the more prevalent mode, and it presents two types according as the act of 
deposition is very rapid, as in the Cottoids, Discoboli, and certain Blennies ; or it is slower, 
as in the Gadidae, and may be even prolonged, as appears to be the case with Trigla. 

The difficulty of ascertaining the real facts in regard to oviposition is apparent when 
it is remembered that, about even so familiar a form as the salmon, opinion has been 
up till comparatively recent times divided; the fishermen being of opinion that the 
process is gradual, and may occupy many days, whereas there is much evidence to show 
that the ova are discharged all at once, or very rapidly. In stripping a ripe female the 

* J. A. Ryder (Atner. Nat., vol. xx. p. 987) states that the periblast is hypoblastic, and that the only source of 
the nuclei in the pigment-cells of the oil-drop must be periblast; therefore these cells are hypoblastic in origin. 
Kingsley and Conn severally observe (op. cit., p. 188) that possibly the ova of all the Gadidse have one or more 
conspicuous oil-globules in the deutoplasm. 

t Op cit., p. 16. He also thinks the perivitelline space has a phylogenetic significance. 


eggs run out with little or no pressure, and the ovaries may often be thus emptied in a 
few moments. Now if the ovaries of a female Cyclopterus lumpus, exemplifying rapid 
deposition, be examined, we find, when in a ripe condition, that the contained ova 
apparently become mature simultaneously.* In such a case great distension of the 
abdomen occurs, and the eggs are deposited in a single large mass in a very short time. 
In a female Cottus scorpius under observation, and likewise distended with ripe ova, 
oviposition occupied only a few seconds. 

A very different condition obtains in other forms, such as Molva vulgaris or Pleuro- 
nectes Jlesus, in which a large proportion of the ova ripen together, yet the act of extrusion 
is more deliberate and slow; while in Gadus morrhua, or more distinctly in Trigla gur- 
nardus, the eggs reach maturity by successive strata, a comparatively small proportion of 
them being ripe and translucent. The latter generally pass posteriorly, and collect near 
the genital opening — ready for extrusion, t Isolated ripe ova, however, are scattered 
throughout the ovaries, and in such forms the extrusion of all the eggs in a single female 
must extend over a prolonged period. 

While the ova remain in the body of the fish they are bathed in a mucilaginous fluid, 
so that they easily glide over each other, and thus their egress is facilitated. This ovarian 
mucus seems to have different properties in different species of Teleosteans, either dis- 
appearing on mixing with water, as we see especially in the non-adhesive floating eggs of 
the cod, haddock, whiting, ling, gurnard, skulpin, flat-fishes, and also in the demersal eggs 
of the Salmonidse, or remaining glutinous and adhesive for some hours — the eggs clinging 
strongly together and forming irregular spongy masses, as in British Cottoids, Discoboli, 
various species of Gastrosteus, as well as the recently discovered ova of Anarrhichas. In 
Lepadogaster,\ however, the ova are fixed singly to shells, sticks, sea-weeds, and other 

After submergence in sea water such ova become so strongly cemented that some 
force is required to separate them, and the egg-masses of forms like Cyclopterus adhere so 
firmly that many of the ova are usually injured in dislodging them. Whether the 
mucilaginous nexus which binds ova like those of Lophius piscatorius together in 
considerable masses, or forms a thick, tenacious layer outside the zona radiata in eggs 
such as those of Perca Jluviatilis, be really an excessive secretion of the mucus spoken of 
above or not is undetermined. 

Demersal ova appear to be deposited by the female on the very sites where the whole 
course of development, up to the time of hatching, will be undergone. With pelagic ova 
the case is very different ; during development they may wander far from the place of 

It must be noted, however, in the case of the cod, and other food-fishes, that the 
grounds upon which the adults congregate are those where the surface specially abounds 
with their pelagic ova, as Sars first noted at Lofoten. 

Upon extrusion the buoyancy of pelagic ova is strikingly shown, for, if ripe, they at 

* Vide Nature, June 1886. t Vide No. 104, p. 363, &c. J Vide No. 106, p. 434. 

VOL. XXXV. PART III. (NO. 19). 5 T 


once ascend like minute crystalline globes of oil, and before fertilisation, as well as after, 
they swim freely in the water (No. 11, p. 36, and No. 65, p. 450). Not only are these 
pelagic ova found at and near the surface of the sea, but, in many areas, throughout the 
greater part of its depth. Moreover, they occur in great numbers near the bottom. In 
calm regions they congregate near the surface in scattered groups, and show no tendency 
to adhere together, save in such exceptional instances as those before mentioned. The 
slightest agitation scatters them, and they are carried to and fro by the currents in the 
surrounding medium.* In very still water in tanks they often form layers extending over 
a considerable area, the lower strata sustaining by their buoyancy the superimposed 
layers, which are even and regular to a remarkable degree (PI. I. fig. 10). Their 
buoyancy is readily affected by a variety of conditions, especially by adulteration of the 
sea water in which they float. In impure sea water t and in fresh water they sink, as 
they also do in alcohol, in which fluid they rapidly become opaque. Dead eggs never 
float, and dying eggs, though remaining translucent, lose their buoyancy. Healthy eggs 
are rapidly affected by unhealthy or putrid ova in their vicinity, a fact showing that the 
zona radiata is pervious, and that endosmosis and exosmosis readily take place, as indeed the 
absorption of water by the partially desiccated ova of the cod (vide p. 681) clearly shows. 

In demersal and pelagic ova unhealthy or dying eggs are readily recognised by the 
opacity of their contents ; and an offensive odour, if the eggs are in masses, indicates that 
they are dead. Small groups of demersal ova, such as those of Cottus and Cyclopterus, 
when dead, may be kept for many weeks in still water in a flat vessel without undergoing 
much change in outline, though of course opacity is complete. 

Fertilisation. — With very few exceptions {e.g., Gambusia patruelis^ Sebastes 
norvegicus, and Zoarces viviparus) the ova of Teleosteans are fertilised after expulsion 
by the shedding of the milt, on the part of the male, in their neighbourhood. The rapid 
diffusion of the milt in water by the serpentine movements of the spermatozoa is very 
striking — they spread through a large area, and in tanks used for artificial fertilisation and 
rearing it is difficult to keep ova in an unfertilised condition if sperms can by any 
possibility find access through the supply-tank. § 

In demersal ova deposited on the sea-bottom, on zoophytes or shells, or (in littoral 
forms) beneath shelving rocks, in hidden nooks of tidal pools, and in some cases in 
nests constructed by the male fish, fertilisation is usually ensured by the proximity of the 
male, which may even carefully guard the ova during development, as is notably the case 
in Cyclopterus lumpus (vide No. 107, pp. 81, 82); but even in this species masses of eggs 
occasionally are found whose fertilisation has not been accomplished. This may some- 
times happen in the case of pelagic ova, though experiments at the Laboratory have 
shown that eggs of haddock may remain for a considerable time unfertilised, and yet be 

* See Hensen's observations proving that pelagic ova are widely scattered in the sea (No. 65, p. 449). 
t Vide No. 104. J No. 141, p. 461. 

§ As occurred to Professor Ewart and Mr Brook at the Rothesay Aquarium, and also with Motella in the St 
Andrews Laboratory. 


successfully fecundated — a series of ova of the species named being fertilised sixteen 
hours after oviposition ; and in the case of the ova of the herring from the deck of a 
fishing boat, G. Brook states that forty-eight hours have been allowed to elapse, yet fertil- 
isation was found to be successful. 

More uncertainty probably exists in the case of pelagic ova, which after expulsion are 
never quiescent, but may travel over large areas, so that at times their fertilisation must 
be a matter of chance. The fishes at the spawning season congregate, it is true, in vast 
numbers, males and females thus herding together ; but ripe females may occasionally 
shed their ova where it is problematical whether sperms will ever reach them, and. in this 
way we can account for the quantity of dead eggs of plaice and cod which Hensen found 
while dredging in the inner bay of Kiel (No. 65, p. 429), though changes in the nature of 
the water have also to be taken into account. If no spermatozoa reach them within a 
limited time after extrusion, pelagic eggs lose their glassy transparency, and descending to 
the bottom assume the white opacity and wrinkled appearance of dead ova. In demersal 
forms, with a denser capsule, the unhealthy or dying condition is not so readily seen; 
but opacity of the contents, and especially an increasingly offensive odour, if in masses, 
are unmistakable indications of loss of vitality.""" 

The relation of the micropyle to effective fertilisation has been already treated of ; 
and many authors regard its position as of the highest importance. Gerbe, indeed, 
satisfied himself in the case of the trout that this position is always superior, and 
he took pains to secure this condition when performing artificial fertilisation (No. 57, 
p. 330). In the uppermost segment he found after fertilisation that a granular layer is 
formed by a process of thickening, so that a " nuage vague " condenses as a circular area 
always in relation to the micropyle. Gerbe would extend the observations he noted in 
the trout to the ova of Teleosteans in general, and certainly in many demersal forms the 
blastodisc concentrates in the uppermost segment, and. the micropyle is stated to be 
uppermost ; yet in pelagic eggs the disc would appear always to be formed at the inferior 
pole, and in such eggs if the constancy of the position of the micropyle be well founded, 
it must be no longer uppermost, but on the under side of the egg, and such is affirmed to 
be the case, though there are difficulties in the way of such an affirmation, and many 
reasons for holding that the position is not necessarily constant. 

Demersal ova do not show a uniformity in the situation of the micropyle, for in the 
egg-tubes of Perca it is not uppermost, but directed to one side, so that it opens into 
the lumen of the cylinder; and Gerbe found that it occupies a like position after 
fertilisation in Salmo fario, the capsule he says, moving through a quadrant, so that the 
micropyle is no longer uppermost ; " this change simply alters the respective positions of 
the cicatricula and the micropyle, and when accomplished the phenomenon to all intents 
and purposes is ended" (No. 57, p. 331). 

* The ova of Osmerus eperlanus would seem to become opaque very rapidly, for Cunningham notes that the 
unfertilised eggs sank to the bottom, remained unattached and free, and became opaque-white shortly after expulsion, 
though at first they were of a translucent yellow (No. 49, p. 293). 


The artificial fertilisation of the eggs of osseous fishes is easily performed, it being 
only necessary to apply ripe spermatozoa (PI. I. fig. 9) from the male to mature ova 
placed in water. If ova and the male element be placed in the same vessel of water, the 
process is accomplished in a few moments. The exact mode by which it was really 
accomplished remained unknown until Ransom not only saw and truly interpreted the 
micropylar opening, but watched spermatozoa make their way into the aperture. # " I 
saw," he says (No. 127, p. 461), "an active spermatozooid enter the apex of the funnel, 
and disappear as if inwards ; a quarter of a minute more had not elapsed before the bright 
circle which marks the aperture became indistinct from shortening of the funnel ; during 
the next two minutes I saw three more spermatozooids enter the apex and vanish 
apparently inwards." Notwithstanding the clear and unmistakable observations pub- 
lished by Ransom, the process of fertilisation is one about which much discussion has 
taken place. Kupffer, as already mentioned, has even doubted that the micropyle 
plays any essential part in fertilisation (No. 87, p. 179) ; and Boeck has advanced a 
theory of endosmosis which is somewhat like the explanation Newport put forward in one 
of his earlier treatises, when, having failed to detect in the ovum of Rana any perforation 
or fissure by which sperms could find access to the egg-contents, he said that mere contact 
with the external envelope must suffice for fertilisation, as he never found spermatozoa in 
contact with the yelk-membrane, or even within the substance of the external envelope 
(No. 112, p. 203). This endosmotic theory Newport afterwards abandoned, and adopted 
the opinion which Dr Martin Barry had put forward — in accordance with the views of 
Leeuwenhoek, and Prevost and Dumas (No. 121), that the spermatozoa penetrate bodily 
into the ovum (No. 21, p. 309). Ransom's explicit account decides the matter, the situa- 
tion and structure of the micropyle clearly indicating its purpose, viz., the admission of 
the spermatozoa to the germinal elements within the ovum (No. 127, p. 462). G. Brook, 
again, has recently affirmed that in Clupea spermatozoa enter on all sides. The interesting 
question remains as to whether one or more spermatic bodies are concerned in the normal 
fecundation of a single ovum. The presumption that one spermatozoon suffices is 
strong, but there are peculiar difficulties in the case of the Teleostean ovum in actually 
observing the fact. The entrance of these bodies has been watched in many Inverte- 
brates, and one spermatozoon is usually found competent to effect fertilisation, though 
Selenka found (in Toxopneustes variegatus) that while one usually enters, several may 
find access, and normal development still follow. Three or four indeed sometimes enter, 
as Hertwig and Fol observed in the same species, and the separate pronuclei formed by 
each usually fuse with the single female pronucleus ; but they found that subsequent 
cleavage was irregular (No. 66). In Petromyzon Calberla's investigations show that 
one sperm only enters, the enlarged head-portion separating at the outer micropyle from 
the tail which is left behind, while the head penetrates the yolk or rather passes along a 
protoplasmic process, which penetrates the yolk and reaches the female pronucleus at the 
inner extremity (No. 38, p. 464). Kupffer and Benecke, again, found that several sperms 

* Doy£re had previously seen the micropyle in Syngnathus. 


may enter in this form (No. 89). In osseous fishes a similar condition would appear to 
obtain, one spermatozoon being sufficient ; but as this does not plug up the micropyle, 
others may also enter, indeed, Ransom observed this in Gastrosteus. " I watched closely 
one egg," he says, " which was placed with the micropyle in full face, so that the aperture 
at its apex was well seen. Spermatozoa approached and entered the funnel, and 
one was watched till it disappeared, apparently in the direction of the interior of the 
egg, just at the moment when it seemed to occupy the aperture at the apex of the 
micropyle. Immediately after the depth of the funnel began to diminish, and a 
breathing chamber commenced to form ; two or three more spermatozoa were, less 
distinctly, seen playing about in the apex of the funnel as it was shortening ; one of 
them appeared to become still before it vanished apparently inwards" (No. 127, p. 461). 
The exaggerated length of the micropylar funnel, which Ransom describes in Gastrosteus 
as enabling it to dip into the granular discus proligerus, has not been described in other 
Teleosteans, Neither Andre nor Gerbe mention it in the trout, nor does His show it 
in the trout or salmon; while in pelagic eggs the micropylar eminence, though distinct, 
is not by any means prominent (PI. I. figs. 11-14). A lengthened micropyle is indeed 
unnecessary, the mere presence of the spermatozoon within the ovum being the 
essential point. The actual entrance of sperms has been seen in very few Tele- 
osteans. Ransom, as already noted, saw them occupying the external orifice of the 
micropyle, and Andre speaks of observing a sperm apparently entangled, in the micro- 
pylar canal, by the jutting ends of the radial striae, which appeared to him to serve 
for securing the sperm after its entrance (No. 4, p. 201); but there seems to be no 
column of protoplasm facilitating the passage of the sperm from the micropyle to the 
female pronucleus, such as Calberla describes in Petromyzon planeri. The head of 
the sperm in this form separates from its flagellum, and passes along the proto- 
plasmic column, which withdraws from the micropyle (Calberla's aussere Mikropyle), 
and the sperm proceeds through the neck of the column (distinguished as the inner 
micropyle) to the enlarged central termination, where the " eikern " or female pro- 
nucleus is seated. Here conjugation of the two pronuclei is effected (No. 38, p. 458, 
Taf. xvii. figs. 5, 6, 7, and 8). Possibly the preformed discus proligerus may repre- 
sent this column ; and in those Teleosteans in which no disc is formed, the distance 
between the inner orifice of the micropyle and the protoplasmic cortex of the vitellus 
is insignificant. The spermatozoa of Teleosteans seem to be of the ordinary type, 
and show, so far as observations go, little difference in structure — the usual head 
or enlarged portion being distinguishable from the hair-like tail or flagellum (PI. I. 
fig. 9). 

Polar Globules. — The details of the phenomena of fertilisation in osseous fishes are 
probably not unlike those in forms more fully known. Hoffman has described the 
formation of the pronucleus and ejection of a polar globule in Scorpcena, Julis, and 
Crenilabrus, and he states that the globule closes up the orifice of the micropyle, 
and prevents the admission of other sperms after that of the single sperm which 


accomplishes fertilisation."" He, however, states that the extrusion of polar bodies, the 
disappearance of the nuclear spindle, and the aggregation of the germinal area may 
take place independently of impregnation. Kingsley and CoNNt describe and figure 
a polar globule in the egg of the cunner apparently after maturation. The globule 
appeared in the centre of the aster, and passed through the micropyle. List recently 
does the same in Crenilabrus pavo, the body being globular at seven minutes and rod- 
like at thirty minutes.! Ryder noted in the ovum of Gadus morrhua a minute 
granular papilla projecting from the early germ, and looked upon this as representing the 
polar cells derived from the germinal vesicle (No. 141, p. 477). In Trigla gurnardus, 
twenty-five minutes after the addition of sperms, a somewhat cylindrical nuclear body 
has been observed in the superficial protoplasm (PL I. fig. 17, a). It exhibited very 
slow amoeboid movements, and five minutes after it was first noted it had shortened 
and contracted in the mid-portion as if dividing into two — a wide granular border 
extending round it (PI. I. fig. 17, 6). Three minutes later the two separating parts 
closely approached, and the body became still more contracted and compact — the 
granular margin also becoming less (PI. I. fig. 17, c) ; but the median cincture was still 
plainly marked ten minutes later (PI. I. fig. 17, d). A side view of a similar structure 
in another ovum exhibited two spherical nuclear bodies enveloped in a vase-shaped mass 
of protoplasm, and from the centre of its wide upper surface radial striations diverged 
(PI. I. fig. 17, e). No similar appearance has been observed in other pelagic ova seen by 
us. Mr Cunningham was more fortunate with the ovum of Plevronectes cynoglossus, 
and he describes a polar globule in this species. § 

The more obvious features in the living ovum after fertilisation are — (1) The 
meridional streaming of the cortical protoplasm to the animal pole. (2) The formation, 
or, in certain forms, the visible increase in the size of the blastodisc, and its assumption of 
a more definite contour. (3) The disappearance of the minute clear vesicles which stud 
the entire cortex of the vitellus, probably as a consequence of the transference of the 
protoplasm to one pole — by which they are carried to the region of the disc. In many 
forms a change in the optical appearance of the yolk is seen. Ransom noticed this, and 
says that the increased clearness and translucency of the yolk is in part due to distention 
and greater transparency of the enveloping layer (No. 127, p. 458); indeed, the whole 
ovum after fertilisation assumes a brighter and more tense appearance. Finally (4), a 
space slowly becomes apparent between the vitelline globe and the inner surface of the 
zona radiata, so that the egg-contents are no longer closely applied to the capsule, as in 
the unfertilised ovum. 

Probably the foregoing features mark the fertilised condition in all Teleostean ova ; 
but there are many forms in which, for various reasons, they cannot readily be discerned. 

* This closure of the micropyle is perhaps incomplete, as the subsequent formation of a perivitelline space is 
due to the entrance of water in the main through the micropyle, though it may also enter by the general surface, 
t Loc. cit., footnote, p. 190. J Op. cit., p. 597, fig. ii. d. 

§ Op. cit., p. 131. 


Especially is this the case in ova which show a preformed discus proligerus. In the pike, 
for instance, Lekeboullet states that both are alike, save in the formation of a " disque 
huileux " which collects in the fertilised egg, as Gerbe also describes in the egg of the 
trout, two hours after fertilisation, the circular germinal area appearing as if enclosed 
in a "crown of oil-globules" (No. 57, p. 330) ; yet even this feature may appear in the 
unimpregnated egg, and it cannot therefore, as Lereboullet confesses, be traced to the 
action of the sperm (vide No. 93, p. 478). The fertilised ovum in pelagic forms (e.g., cod 
and gurnard) is more readily distinguished, as the segregation of the protoplasm is plainly 
visible within an hour or two after fertilisation; but the transference is not to the upper 
pole, as in a large number of demersal forms, but to the lower pole, where the patera or 
flattened disc is formed of clear, straw-tinted protoplasm containing minute spherules, 
which are especially numerous at the base and periphery. During the process of segrega- 
tion the contour of the vitellus becomes very distinctly corrugated — an appearance pro- 
duced by the streaming of the protoplasm along definite meridional lines ; and pelagic 
forms are especially favourable for observing this polar transference. Ransom, in common 
with other observers, wholly failed to detect this movement (No. 127, p. 458), though he 
says that the granules often form radial lines round the margin of the concentrating disc 
(Ibid., p. 459). Besides passing along the superficial areas, much protoplasm probably 
also glides in the deeper strata of the vitellus to the base of the germ during the first 
hour after entrance of the sperm. Such streaming of the protoplasm towards the disc has 
been noted by many observers, and recently Kowalewsky has described it in Carassius, 
Polyacanthus, and Gobius (No. 86). In two hours or more, according to the temperature 
and other conditions, a plano-convex disc is formed, composed of an almost homogeneous 
matrix. The disc in the fertilised ovum is always well defined and prominent, and 
continues to receive additions of protoplasm, so that it increases in size, and becomes more 
pronounced; whereas in the unfertilised ovum, when a disc is formed, it becomes "vague, 
irregular in outline, and loses coherency" (No. 57, p. 330). The primary segmentation- 
nucleus has rarely been detected in the blastodisc before cleavage, granules and colourless 
vesicles alone appearing in its matrix. The breathing chamber gradually becomes more 
distinct ; but this may also happen in the unfertilised condition, as Ransom found that such 
ova may, after being in contact with water for an hour, show this marked interspace. Its 
formation, as well as the concentration of the disc, Ransom holds to be only indirectly 
due to the spermatozoa, which may render more easy and rapid the influx of the 
surrounding medium into the egg (No. 127, p. 463). The same observer carefully studied 
the formation of this space in Gastrosteus, and states that it first appears close to the 
micropyle, whence it " gradually extends over the rest of the yolk-ball, being complete in 
three to five minutes after the spermatozooids have been applied" (Ibid., p. 457); but in a 
note at the foot of the page he says that water may enter more freely, and the chamber 
arise simultaneously in the ova of other fishes. Newport, who was the first to signalise 
this perivitelline space, speaks of it as " respiratory," and being in Rana " at first but a 
small area" (No. 112, p. 187), a view coinciding with Ransom's upon the same ovum, for 


he believed he saw it arise, just as in Gastrosteus, near the micropyle. Most recent 
observers, including List, describe this perivitelline space in Teleostean eggs. To 
what is the formation of this chamber due % Does the vitellus, which before fecundation 
fills up the intra-capsular area, diminish, or does the external capsule really enlarge ? On 
the one hand, Ransom maintains that the yolk-sac or capsule enlarges (No. 127, p. 457); 
while, on the other hand, Gerbe believes that, by the contraction of the vitellus, this 
" zone of separation " is produced (No. 57, p. 330). Keber has further surmised that part 
of the contents of the egg may flow out through the micropyle (No. 77), and the egg- 
mass would thus decrease. Kupffer considers that both the first mentioned phenomena 
happen, for he says that in Clupea not only does the yolk contract, but the capsule 
enlarges by as much as one-quarter of its diameter (No. 87, p. 185). A still more 
marked increase in size Lereboullet noted in the egg of Perca, which, he says, by 
absorption of water through the radial tubes acquires a volume twice that which it had 
before extrusion (No. 93, p. 471). Usually, however, the enlargement of the Teleostean 
ovum is so small as not to be readily noticed. 

Movements of the Yolk. — The curious movements of the vitelline mass, which have 
been described by many observers, and are stated by Ransom to be " the most striking 
phenomena which follow on the entrance of the spermatozooids into the egg" (No. 127, 
p. 463), are not visible in all Teleostean ova. At any rate, if performed at all, they are 
obscure, or so imperceptible as to have escaped notice in pelagic ova, while in demersal 
ova they are occasionally not exhibited — Lereboullet indeed affirming that in Perca 
jluviatilis the egg-contents remain unmoved, and at no time show the intra-capsular move- 
ments so remarkably distinct in Esox (No. 93, p. 503). He further says — " I have not 
seen it (the rotatory movement) in the white fishes, of which I have observed many species, 
and M. Vogt has not noticed it in Coregonus." In addition to the undulations, or 
" oscillations " as Ransom terms them, which usually pass like a wave of contraction from 
one pole * to the opposite pole, and occasionally along the equatorial line, producing a 
dumb-bell outline in the latter case, there are rotations of the vitellus en masse. Ransom 
did not observe any rotation in Gastrosteus, which exhibits the oscillations very distinctly, 
nor did he in other ova, though he admits that such movements on the polar axis were 
not improbable. Lereboullet again speaks of another movement, in fact, a simultaneous 
double movement: the vitellus, he says, " exerce un mouvementde rotation sur son axe et 
un mouvement de translation autour de la coque " (No. 93, p. 497). These motions seem 
to continue during the early progress of cleavage, but cease, according to Lereboullet, when 
three-quarters of the yolk-surface are enveloped. He describes at this later stage, in 
Esox, an alteration in the form of the vitellus ; it elongates and becomes pear-shaped, the 
narrowest diameter circumscribing the part of the yolk not yet covered by the extending 
blastoderm. Bambeke, in Leuciscus (?), described the same change of shape, and speaks 
of the opening (the blastopore, or trou vitellaire of C. Vogt) as resembling the mouth of 

* Ransom says the pole at which the movements commence is that resting on, i.e., in contact with, the capsule ; but 
this can hardly be so. 


a balloon (No. 20a). The change of outline Bambeke attributes to epiboly, the blastoderm 
squeezing the fluid yolk out of shape ; and this is not improbable, for in various pelagic 
ova the naked yolk, i.e., that part not yet covered, projects boldly from the blastopore 
like a plug pressed out from the diminishing aperture. The change in shape 
might be attributed to the contractility of the yolk — an inherent property according to 
Eeichert (No. 134) ; but there is much reason for holding that the active agent is the 
amoeboid protoplasmic cortex, or the blastoderm itself external to that layer. A most 
remarkable phenomenon was observed by Lereboullet in the ovum of Esox at the stage 
just referred to, when the usual rotation is perceptibly diminishing, for he states that 
the blastoderm seemed to continue its rotation " as if disconnected from the yolk, and 
the latter continued to turn from right to left as though inside a loose sac" (No. 93, 
p. 491). 

What the significance of these varied movements really is cannot be definitely stated ; 
but that they are connected with the separation of the germinal matter from the food- 
yolk proper, as Ransom surmises, seems very probable. Ransom, indeed, would go further, 
and regard them as a form of contractile movement, not remotely connected with seg- 
mentation (No. 127, p. 495); and it is noteworthy that these movements cease when the 
germinal matter has, for the most part, separated from the trophic element in the 
vitellus. The yolk alters its form soon after fertilisation, as Lereboullet observed in the 
pike ; and he refers to a movement of the constituent elements of the egg — the marked 
flattening of the spherical yolk, which now becomes elliptical (see No. 127, pi. i. fig. 17), 
while the blastodisc projects prominently from its surface. 

Whether the yolk-matter itself, or the protoplasmic envelope outside, really produces 
the rhythmic contractions referred to, the phenomena depend, as Ransom found, upon the 
presence of oxygen in the surrounding medium, while carbonic acid produces total cessa- 
tion or a marked repression of these movements (No. 128, p. 237). They seem to demand 
less oxygen than cleavage proper (No. 127, p. 495), though the amount of oxygen used is 
small; and Ransom did not succeed in obtaining chemical evidence as to the products 
of the oxidation which undoubtedly goes on. 

The conclusion that all the movements collectively known as yolk-contractions are 
connected with the polar segregation of the germinal protoplasm, is probably near the 
truth. That their existence, or at any rate their vividness, is correlated to peculiarities in 
the early development of the germ there is no proof, and Ransom's conclusion is very 
much at variance, indeed, wholly opposed to the facts, when he says that such movements 
in Esox and Gastrosteus are connected with rapidity of development (No. 127, p. 495). 
These forms, instead of hatching in a shorter time than those with slow or indistinct con- 
tractions, have an embryonic development unusually prolonged, so that the reverse of the 
above conclusion is really true, viz., that the ova in which these movements are not 
merely indistinct but imperceptible, are of all forms the most rapid in development, 
and of such rapidly developing eggs those of the Gadidse and Pleuronectidse are marked 

VOL. XXXV. PART III. (NO. 19). 5 U 


IV. Segmentation. 

At the time segmentation begins (always within a few hours after fertilisation) 
the process of segregation is to a great extent completed, and the germinal disc is 
defined as a thickened patera of clear protoplasm lying upon the yolk in those forms 
whose upper segment is the animal pole, or depending from the yolk in those ova 
with an inferior animal pole, and separated by an intermediate stratum, which differs 
both from the yolk and the germinal matrix. Thus ova of the haddock, fertilised at 
2 p.m. on 23rd March 1886, showed at 8.50 p.m. a uniform prominent mass or cap of 
protoplasm without trace of segmentation. At the margin were numerous protoplasmic 
processes, rising in some cases on the surface of the yolk into globules. On the second 
day the rim of some of the granular spheres projected beyond the disc at the lower pole. 
Whether the separate cells, seen during development, in the perivitelline space are 
due to these projections is unknown. In the cod, again (see PI. X. fig. 9), the spheres, 
which differ in size, show minute granules. The nuclei of the spheres are not always 
easily seen in the living egg, but with due care can generally be made out. 

Eyder is right in saying that the cleavage does not at first go quite through the disc, 
the contrary being stated by Kingsley and Conn. The latter authors noticed marked 
amoeboid movements at the 4- and 8-celled stages, processes being sent out by the spheres. 

In the early stage of segmentation the Teleostean egg shows external larger spheres 
and internal smaller ones (PI. IX. fig. 8), just as Janosik* found in Crenilabrus and 
Tinea, the internal dividing more quickly than the external. This, likewise, is observed 
in the Elasmobranch egg. 

We have seen that all the features of the fertilised ovum may appear to some extent 
in the unfecundated egg, and though segmentation is usually an indication that fertilisa- 
tion has taken place, it is not infallibly so. Oellacher found cleavage-lines passing 
across the germ in an unimpregnated egg of the fowl (No. 113),t and in Teleostean ova 
the disc may break up into segments by an irregular kind of cleavage. Its abnormal 
character is soon revealed, resembling as it does the cleavage of unhealthy and dying 
eggs, the cells always showing great irregularity, and the protoplasm composing them 
assuming a more or less marked opacity or a granular appearance. Both in size and 
shape Lereboullet found that these abnormal cleavage-segments differed from the 
normal (No. 93, p. 485). 

The Cortical Protoplasm. — The blastodisc is formed by the segregation at one pole of 
protoplasm, which, moreover, constitutes a superficial and tenacious layer around the 
vitellus. This layer is itself derived by centrifugal transference from the scattered proto- 
plasm mingled with the general matrix of the yolk, a phenomenon which recalls the 
formation of the periblastula in the crustacean ovum, such as that of Astacus. In this ovum 

* Archivf. Mikr. Anat., vol. xxiv. 

+ Bischoff (Ann. d. Sci. Nat., iii. se>., ZooL, t. ii.), Hensen (Centralblatt f. die Med. Wiss., 1869), Kidd (Quart. 
Jour. Micr. Sci., xvii., 1877), and others have confirmed Oellacher's observation in other forms, especially Mammals. 


the protoplasm interfused with the yolk also collects at the surface, though it is not 
visibly separated by a line of demarcation, and can only be recognised by its texture and 
property of readily staining. Ere long it completely separates from the granular deuto- 
plasm, and forms a superficial blastodermic layer enveloping the yolk.* In the same 
manner a protoplasmic cortex, like the periblastula just mentioned, forms an equal layer 
over the yolk in fishes' eggs, but is not at first sharply defined, though later it is so. 
Balfour observes that in Elasmobranchs the disc is merely a part of the ovum in which 
the protoplasm is more concentrated, and the yolk-spherules smaller than elsewhere. 

In the ova of the haddock on the second day the blastodisc shows small " oil-globules " 
amongst the protoplasm between the spheres, and the disc presents a pale salmon-tint by 
transmitted light. Usually it appears to consist of homogeneous protoplasm, with numerous 
small spheres of oil or indifferent fluid and scattered granules. In Clwpea harengus no 
cortical layer is present before segmentation, according to Kupffer (No. 87, p. 179), nor 
is a blastodisc preformed, this latter feature being shown also by Gadoid and other 
pelagic ova, though in these eggs a cortical layer is well defined before fertilisation. 
Notwithstanding that the cortex seems thus sharply marked off from the yolk, there is 
good reason to believe that the centrifugal movement of the deeper interfused protoplasm 
does not cease when the layer is formed, and Klein refers to this process as the feeding 
of the cortex upon the yolk for purposes of growth (No. 79). Balfour also speaks of 
certain nutritive elements of the yolk as being converted into protoplasm (No. 11, note 
at foot of p. 679), and Kupffer (No. 88, p. 214) and Rieneck (No. 137) have adopted a 
similar view, as also more recently has G. Brook (No. 30). 

No nuclei can be detected in the cortex ; but clear structureless spheres occur in 
small groups, or singly over its surface, and these coalesce later, and form larger spheres, 
which are found at the base of the blastodisc during segmentation. Ryder has 
determined their composition to be that merely of an indifferent fluid (No. 141, p. 467). 
Outside this cortical protoplasm Ransom distinguishes a delicate homogeneous layer, his 
" inner yolk sac," which is not possessed by the more immature eggs. In "the smallest 
intra-ovarian ova " examined in saliva, he says " the yolk is granular and irregular, not 
smoothly defined as it would be were an inner sac present " (No. 127, p. 442) ; and in ova 
two-thirds their full size, also, he failed to perceive it. When intact it seems able to 
resist osmotic currents in Salmo solar, and it varies in bulk, being unusually thick in the 
ruffe (Acerina) (Ibid., p. 453). 

Such an inner-sac would appear to be absent in Gadoid and similar pelagic ova, 
and indeed in the forms studied by Ransom the precise nature of the so-called inner-sac 
is a subject for further investigation. He regards it as a membrane, as performing 
contractile movements, and as folded in along the lines of blastodermic cleavage 
(No. 127, p. 479). 

It is difficult, however, to conceive a structure, meriting the name membrane, envelop- 
ing yolk and germinal disc so closely as to be almost inseparable, and involved in the 

* Vide Reichenbach, "Die Embiyonalanlage und erste Entwickelung des Flusskrebses," Zeit. f. w. Z., xxix., 1877. 


cleavage-process. The view that it is simply the cortical protoplasm, and not a definite 
membrane (vide No. 122, p. 445), is supported by certain facts which Ransom mentions, for 
he speaks of the inner face of the yolk-sac as ill-defined and closely connected with the 
formative yolk (No. 127, p. 433), and that on rupture the shreds change their form 
(Ibid., p. 478) and are frequently drawn out into thread-like prolongations (p. 468) ; 
while he further describes it as continuous with the blastoderm (p. 467), and admits 
that, as it ultimately shares in the cleavage process, it " may to that extent be considered 
a part of the formative yolk" (p. 433) or germinal protoplasm. The presence of a 
like membrane investing the germ has been maintained by Schenk in the ovum of 
Elasmobranchs (No. 142), but other observers, including Leydig and Balfour, have 
denied its existence. The yolk-sac described in the hardly mature ovum of Rana by 
Cramer (No. 45, p. 33) as a distinct membrane before cleavage begins, is merely the 
more consistent superficies of the yolk-ball, and not a separable structure. The fact 
seems to be that what Ransom regards as a distinct membrane is the cuticular stratum 
of the protoplasmic cortex, and is therefore less of the nature of a sac than that of an 
external layer, slightly more consistent than the protoplasm underneath. Ransom 
admits that in a sense it may be so regarded (No. 127, p. 433); and it is adherent to 
the blastodisc, over the outer surface of which it passes, and probably constitutes the 
clear matrix, as distinct from the granules of the disc. It forms folds at the margin of 
the clefts during segmentation, " reminding one," he says, " of the ' Faltenkranz,' — 
described by Reichert and by Schultze in the frog's egg," — these folds being in fact 
the familiar corrugations produced by the cleavage and separation of the blastomeres. 
Sections through the disc at this time show no investing membrane, though it is true 
that the cortex takes a slightly deeper stain than the underlying matrix of the 
blastomeres, but the one insensibly passes into the other. Balfour also found, in the 
ova of Elasmobranchs, that the surface was very susceptible to stains, and that the sides 
of the furrows took a deep colour ; but such appearances did not suffice, in his view, to 
demonstrate a separate membrane, so that in Teleosteans, also, we must, with Lereboullet, 
affirm "l'absence de membrane propre " (No. 95, p. 13) outside the blastoderm. That 
Ransom's layer is simply the cortical protoplasm is shown by the fact that on rupturing 
it no coherent layer beneath held in the contents, but the food-yolk immediately flowed 
out (No. 127, p. 465). Ransom himself also speaks of the formative yolk as a layer invest- 
ing the yolk-ball. We cannot, therefore, recognise an inner yolk-sac as such, for the 
somewhat viscid and coherent layer, which alone appears to envelop the yolk, would 
behave precisely as Ransom's yolk-sac did, when in contact on its inner side with the 
semi-fluid yolk, and on its outer side with the watery perivitelline fluid. The whole 
of this cortical protoplasm, however, does not enter the blastodisc and undergo seg- 
mentation ; a considerable part never reaches the animal pole, but permanently clothes 
the yolk-globe, and part of it may temporarily form a supplementary disc at the 
vegetal pole, as Kupffer saw in Clupea (No. 87, p. 185) ; while a portion remains as a 
sub-blastodermic stratum, and becomes thickened as a peripheral wall, the nuclear zone, 


or periblast proper, around the margin of the disc. A thin stratum may also be 
distinguished creeping over the segmenting blastoderm as an external pellicle, referred 
to before as probably homologous with Ransom's inner sac, and this layer sends down 
processes which fill up the interspaces between the large primary blastomeres 
(PL II. fig. 1, p). This appearance, which is distinctly seen in sections of the early 
blastoderm, may, it is true, be really the dilute plasma, or perivitelline fluid, penetrating 
the inter-blastomeric fissures, though more probably it is periblastic protoplasm, forming 
an intermediary substance, such as Lereboullet distinctly recognised (No. 93, p. 493), 
and as E. van Beneden figures (No. 25, pi. iv. fig. 7, &c). 

To sum up briefly, we may say that the protoplasm interfused with the food-yolk 
continues from a late intra-ovarian stage to collect superficially as a cortical layer, and 
forms — 

(1) The blastodisc at the animal pole, and in rare cases a transient pseudo-disc at 
the vegetal pole (PI. II. fig. 1, bdm). 

(2) The intermediary, or sub-blastodermic layer (PI. II. fig. 1, p 1 ). 

(3) The thickened marginal wall or periblast-ring (PI. II. fig. l,per). 

(4) The superficial envelope and inter-blastomeric substance of the segmented disc 
(PI. II. fig. l,p 2 ). 

(5) The sole intra-capsular envelope of the deutoplasmic globe or yolk, prior to the 
epibolic extension of the blastoderm (PI. II. fig. 1, p z ). 

The Subgerminal or Nutritive Disc. — Reference has been made to the layer of proto- 
plasm beneath the blastoderm proper (PI. II. figs. 1 and 15, a, b, c, d, e — cp), and it has 
been distinguished from the periblast proper, i.e., the thickened peripheral wall, and the 
nuclear zone round the margin of the disc, by various names, such as " intermediary layer" 
(Bambeke), " disque huileux " (Lereboullet), " Rindenschicht " (His), " median lens or 
lentille " (E. van Beneden); while other observers, e.g., Haeckel and Ransom, have not 
recognised it, the latter indeed saying of the blastodermic surface in contiguity to the 
yolk, that it seems to be merely " the corpuscles resulting from segmentation in contact 
with the fluid-yolk" (No. 127, p. 467). It appears to arise like the rest of the protoplasmic 
envelope of the yolk by superficial segregation, though Bambeke attributes its formation 
to a centripetal extension of the peripheral annulus ; but Lereboullet's statement 
probably represents the origin of this sub-blastodermic stratum more truly, when 
he says that in Esox and Perca it arises simultaneously with the disc, these nutritive 
elements, as he calls them, following the plastic element in their migration to the 
animal pole (No. 93, p. 11), and at the earliest stages may, as Kupffer supposes, give 
nutriment to the germinal disc (No. 87, p. 194). Ransom did not distinguish a stratum, 
however, but speaks of " a collection of dark oil-granules distinct from the large drops 
which float in the yolk." He saw granules and globules of oil below the disc, and as 
these are consumed during the development of the germ-mass, it is probable that a 
kind of yolk-digestion goes on. 


Lereboullet, Kupffer, Rieneck, and Oellacher all noticed the accumulation of 

globules under the disc in impregnated ova ; and Bambeke (who quotes them) says these 

indicate food-particles for nourishing the germ. Gerbe figures a crown of oil-globules 

around the periphery of the disc (No. 57, p. 330, pi. xii. figs. 3 and 4, b) ; while 

Oellacher speaks of his lenticular germinal mass as including a lower layer which 

imprisons many oil-spheres, and at times is seen to be separated by a distinct contour 

from the disc. Oellacher regards it as part of the blastodisc, and Bambeke likens 

it to his intermediary layer, though the subgerminal disc has been distinguished 

as a separate structure, neither to be confounded with the lower part of the germinal 

disc nor with the intermediary layer. Lereboullet indeed distinctly affirms that 

his mucous layer underlies, as a definite membrane, the blastoderm, while it rests 

upon the nutritive disc. Bambeke erroneously likens his intermediary layer to this 

stratum beneath Lereboullet's mucous layer in the trout (No. 20a). In Lereboullet's 

view, three distinct strata must be recognised at the animal pole — (1) the germinal 

disc proper, (2) the mucous or intermediary layer, and (3) the " disque huileux " or 

nutritive layer. The separation of the stratum underneath the disc into two layers has 

caused some confusion, and the distinction is perhaps unnecessary. It is readily seen 

that the lower portion of the intermediary layer will be more fully charged with oily 

spherules and granules from the yolk than the portion in apposition to the base of the 

disc, but it is needless to separate it as a distinct oily stratum. A subgerminal stratum 

is probably not absent in any Teleostean ovum, though less prominently seen in some 

(e.g., Gadoids and Pleuronectids) than in others (Esox and Gastrosteus), but the 

presence of a layer beneath the subgerminal stratum has been noted by very few 

observers. We cannot indeed regard Lereboullet's lowest (third) layer as separate from 

his mucous layer, which has been so generally recognised in Teleosteans. This single 

subgerminal layer, in whose lowest stratum oily spheres and granules are numerous, is the 

granular layer which Balfour speaks of, though in Elasmobranchs it consists chiefly of 

small yolk-spherules, and it is also Gotte's floor of the germinal cavity (the 

" Dotterzellen"). In Teleosteans it is continuous with the peripheral wall of protoplasm 

(His's "Keimwall") and the thin periblast beyond, originating in the same way, and 

persisting probably by continual renovation, the blastoderm thus feeding upon this 

finely granular layer. Kowalewsky regards the intermediary layer as a provisional organ 

(op. cit., 1886). We call by the name "subgerminal or nutritive disc" the disc-like 

stratum beneath the germ, and it embraces Lerebodllet's two layers — the mucous and 

the oily stratum ; it is the thin central part of Bambeke's intermediary layer ; it is 

Oellacher's inner layer, holding many oil-globules, of the " Eindenschicht ;" and although 

Oellacher speaks of it as more coarsely granular than the disc or layer above, yet 

it is derived from it. Oellacher rightly compares his lower layer to Lereboullet's 

mucous layer ; while Bambeke also correctly says that both are really his intermediary 


We can therefore distinguish (with Bambeke) at the animal pole only two strata — 


(l) the blastodisc, or true segmenting mass ; (2) a granular layer, or subgerminal disc not 
segmenting, and probably nutritive, and interposed between No. 1 and the vitellus. 

V. The Blastoderm. 

Within one or two hours after the entrance of the spermatozoa, the thickened cap 
of protoplasm, either preformed as a discus proligerus, or segregated as a blastodisc 
proper, undergoes segmentation. The blastodisc is readily distinguished with the 
naked eye in the more transparent ova as a spot of lighter colour than the yolk 
on which it is placed ; while under a lower power it is seen protruding as a discoid 
prominence at either the upper or the lower pole, according to the particular form. 
In certain Salmonidse, for instance, the germ always floats uppermost, as it also does in 
the sterlet, according to Salensky, and in the trout ; this being due, according to Ransom, 
to the oil attached to the disc, which compels it to float in the upper segment (No. 127, 
p. 450). # In a number of pelagic ova, possibly in all, the disc lies underneath the yolk, 
the animal pole being inferior ; but whether superior or inferior, the position is constant 
for the species, and there is no actual reversal, such as occurs in Cephalopods, where the 
germ and the yolk-pole exchange places at a certain stage. As the vitelline mass revolves 
freely in the peri vitelline fluid, the germ may often be brought to the upper side by 
agitation in the water ; but it usually seeks the lower pole at once, and remains there 
when the egg is at rest. 

Balfour views the disc merely as a part of the ovum, which is characterised by the 
presence of more protoplasm than the rest of the vitellus (No. 10, p. 106); but while 
this is so in the Elasmobranch and Amphibian ovum, in the Teleostei the germ is so well 
marked and distinct, and, with the exception of some colourless vesicles and a few granules, 
so destitute of yolk-matter (apparently consisting of pure protoplasm) that the yolk 
becomes rather an appendix than an essential part of the germ. 

The same author supposed that the Teleostean yolk at some later stage must be almost 
entirely deprived of the protoplasm so abundantly interfused during the early stages, 
and this undoubtedly is so, the yolk-matrix before it wholly disappears increasing in 
density and coherency.t That the disc owes its origin to fecundation in all Teleosteans, 
we have seen to be an error ; and the view of Coste, which Lereboullet adopts (No. 93, 
p. 33), is not more tenable — viz., that the disc is derived solely from the divided and 
scattered germinal vesicle — for, in some species, the discus proligerus is formed and this 
vesicle is seated in its midst. As the segregation of the disc proceeds, and its mass 
increases, its colour likewise becomes deeper ; and Ransom believes that it undergoes a 
physical change, " being more solid " than in its earlier condition. 

The disc then is the essential part of the ovum, and the yolk is merely supplementary, 

* His figures the germ disc of Esox as uppermost (No. 67, Taf. i. fig. 13); but Lereboullet says, " Sa position est 
oblique ou, si Ton vent inclined a l'equator " (No. 93, p. 481). 

t In a form like Anarrhichas the embryo remains long (several months) within the ovum, and when treated with 
alcohol the yolk becomes extremely hard, and apparently consists of a purely albuminoid matrix. This likewise is the 
case with the ovum of Salmo salar. Sea water also hardens the yolk of the latter species (vide No. 104a, p. 153). 


though the view is held by many authorities (Van Beneden, No. 25, pp. 52, 53; Hoffman) 
that segregation is equivalent to cleavage, and that when the disc is defined the ovum 
consists of two cells — one being the germ, and the other the yolk. The behaviour 
and undoubted function of the deutoplasmic globe is opposed to this view, the separation 
of the germinal matter from the inert yolk being protracted and undefined, and wholly 
unlike cleavage. Nor in the syncytial yolk has a nucleus been discovered equivalent to 
the segmentation-nucleus formed from the fusion of the male and female pronuclei in the 
germ. Dr Martin Barry, half a century ago (No. 21, p. 313), noted in the ovum of 
Rana a nuclear body, which he described as elliptical, well defined in contour, apparently 
granular, and placed within the membrana vitelli [vide Barry's figure, No. 21, pi. vi. fig. 
28, d), but no such additional nucleus is apparently present in the Teleostean yolk. # The 
emphatically passive and inert character of the Teleostean yolk has already been indicated, 
and the real distinction of the active germ from its trophic appendage insisted on. 
We have referred to the relation of the early blastomeres and the potential yolk-segments 
Cunningham speaks of; but however plausible that view may appear during the first 
stages of cleavage, it is difficult to maintain such a relation of blastomeres and yolk 
when the morula is reached. The disc indeed becomes disengaged from the yolk (Gerbe 
says it completely separates, No. 57, p. 330), and a series of independent phenomena begin 
which concern it alone. We do not now allude to the formation of a true cavity beneath 
the disc, as this phenomenon falls to be considered later, but to the embryological 
separation between the germ and yolk, when their physical relations are most intimate. 
Cunningham (No. 48), referring to the statement made by Agassiz and Whitman 
(No. 2) that this separation dates from the 16-cell stage, observes, with greater accuracy 
than the two authorities named, that this separation by a cavity is not seen in living 
ova at the centre of the disc, and sections prove Cunningham to be right. In sections 
the line of demarcation is broken by knob-like processes which project from the 
blastoderm into the yolk (PI. II. fig. 1), and these appear to be masses of protoplasm 
in the act of entering the disc, though another interpretation remains, viz. , that they are 
pseudopodial protuberances.t During segregation and early segmentation remarkable 
changes of form are seen in the Teleostean blastodisc — similar to the phenomena Schenk 
noted in Elasmobranchs, and confirmed by Alex. Schultz (see Balfour, No. 10, p. 410), 
consisting of an alternate rhythmical pullulation and subsequent flattening or subsidence — 
a movement which involves the entire mass of the unsegmented disc (so that it seems 
to draw together and become compact and prominent). This is shared by the individual 
blastomeres in the segmented disc, as the separate cells appear at one time prominent 
rounded bodies standing boldly out upon the yolk, at another time as conical or irregular 
mounds (PI. X. figs. 9, 10), or again flattened structures, crescentiform in section, their 
outline in the last case being less definite, and the entire disc exhibiting a diffuse and 

* See Balbiani, Comptes rendus, 1864, tome lviii. 

t Kowalewsky noted these transition-elements, and says that all stages can be seen amongst the entoblastic(yolk- 
mass) cells forming below the blastoderm — from those which are still in the yolk to those which had entered the 
blastodermic elements, and were only at one point of their bases united to the protoplasmic network of the yolk. 


expanded appearance. These changes of external form, which are often combined with 
an apparent dehiscence of the blastoderm and puckering of the under surface (PL II. 
fig. 14), are probably due to the inherent mobility of the protoplasm; but are also 
connected doubtless with the transference of the cortical matter which has not yet ceased. 
They are especially noticeable when fresh cleavage is about to commence, as Eansom 
seems to have observed (No. 24, p. 466). 

The primary segmentation-nucleus is rarely visible in the germinal disc,* though 
Kupffer noted it as a clear homogeneous vesicle, fifteen or twenty minutes after fertilisation, 
situated in the basal stratum of the blastodisc of Clupea (No. 87, p. 206). In the section of 
the blastoderm of Gadus ceglefinus, at the 5th hour, when two blastomeres are completed, 
we see that the nucleus (n) occupies a position slightly above the basal stratum, and presents 
surrounding radial structures, apparently prolongations of the nuclear substance itself 
(PL II. fig. 18). When this nucleus has divided into two, each is seen to occupy a central 
position in the pair of newly-formed blastomeres. The two blastomeres (PL XXVIII. fig. 4) 
often show disparity in size, with a more or less distinct reniform outline when viewed from 
above. This disparity may be due to unequal segregation of protoplasm, or to more obscure 
causes, but the shape of the earliest blastomeres appears also to depend upon the direction 
of the first plane of cleavage; for, when this is in the shorter axis of the blastodisc, the two 
cells are rudely discoidal, and are in contact by their flattened margin; or if in the longer 
direction, the result, as in the gurnard, is the production of a pair of reniform cells — the 
hilum, so to speak, of each coinciding with the proximal margin. The nucleus in each blasto- 
mere is not spherical, but slightly elliptical and flattened, showing indeed as a transparent 
almond-shaped body, when viewed in profile, and of a paler hue than the surrounding matrix. 
In the living ovum the nuclei are usually very difficult to detect during the earlier stages, 
and Eansom failed to make them out (No. 1 27, p. 467) ; but, when not diaphanous, the nuclei 
may appear, e.g., in the 2-cell stage of Gastrosteus spinachia and Trigla gurnardus, 
as minute irregular vesicles, like clear vacuolations distributed in each blastomere. The 
protoplasm around the central nucleus of each blastomere exhibits a radial disposition 
like the figure of the "lines of force" around a magnet (PL II. fig. 18), but the more 
detailed features of nuclear and blastomeric cleavage are of the complex nature charac- 
teristic of karyokinesis. Each cleavage begins as a superficial indentation, which in the 
case of the first furrow commences in the centre of the straw-tinted pullulation or 
granular blastodisc, within an hour or more from the first appearance of the disc, and 
extends outwards, its course being preceded by puckerings, as though the two masses 
were drawing apart, and producing the beaded structure described by Balfour 
(No. 11, p. 391). The diverging course of the cleavage-plane is not opposed to the 
" loi centripete " of M. Serres, for the plane penetrates (centripetally) the disc. The 
vacuolations which produce the beaded appearance, while most numerous at the margin of 

* Ransom failed to make out the primary segmentation-nucleus, and indeed the blastomeric nuclei. Possibly 
various species may differ in regard to the visibility of the nuclei, for Lereboullet found the nucleus in Perca with 
difficulty, whereas in Esox it was well seen (No. 93, p. 513). 

VOL. XXXV. PART. III. (NO. 19). 5 X 


the cleavage-plane, occur sparsely over the disc, and especially in its basal portion (PL II. 
fig. 18). They probably have an important relation to the cleavage-process, as Balfour 
thought. In sections they occur as clear rounded vacuolations, but are without doubt 
filled with indifferent fluid, and probably are no other bodies than the clear vesicles 
scattered over the cortical protoplasm in the ripe unfertilised egg. The vesicles disappear, 
as we have seen, with the polar segregation of the disc ; but they really persist, and are 
transferred to the disc, where they accumulate (PI. II. fig. 18), often coalescing and 
forming larger vesicles, but not to be confounded with the oily extra- embryonic spheres, 
though Lereboullet does so, saying — " I have seen large transparent spaces like those 
M. Vogt shows in his figs. 113 and 114 (Embr. of Salmon) produced by oil" (No. 93, 
p. 486). It is possible that these vesicles, or rather their clear fluid contents, may render 
mechanical aid during cleavage, filling up with their less consistent matter the furrows 
formed by the dehiscence of the segmenting blastomeres. After the first furrow, which 
is perpendicular to the basal plane of the disc, has produced the first pair of blastomeres, 
the pullulation of the protoplasm is marked, each cell becoming increasingly definite, 
a feature which Kupffer regards as indicating the appearance of an equatorial furrow 
(No. 87, p. 196, Taf. ii. fig. 15, &c). Such an equatorial furrow, according to Hoffman, 
appears before the first perpendicular furrow, and thus the disc would be separated 
from the marginal protoplasm as well as from the yolk at the first stage in cleavage. 
A complete discontinuity of yolk and germ produced by cleavage does not accord well 
with the actual condition in the ovum, and the first furrow would appear to be the 
primary perpendicular one. When this furrow has penetrated almost to the base, 
for it does not perfectly bisect the disc, as Lereboullet long ago noticed (No. 93, p. 481 ; 
see his fig. 18, pi. i.), small furrows directed towards the centre of the disc, appear at 
right angles to the first cleavage-lines, followed by the appearance, along the course 
marked by them, of a second cleavage-furrow, which divides the two primary blastomeres 
into four almost equal segments. Each of the newly-formed blastomeres has a rudely 
square outline, its two free outer sides being rounded, while the two inner sides are more 
nearly straight lines, and mark the perpendicular planes which are in apposition to the 
similar surfaces of the two neighbouring blastomeres. In each blastomere a large nucleus 
can be made out, though often with difficulty, as Lereboullet noted; but not ill-defined, 
as the same author further stated (No. 93, p. 483), for the nuclei appear as homogeneous 
hyaline vesicles with a smooth and distinct contour, the bright contents of which are 
termed by Auerbach the " ground substance " # (PI. II. fig. 4, a). Nuclear division is 
not easy to follow in the living ovum, though blastomeric cleavage is readily observed. 
The ovum of Esox seems well adapted for nuclear observations, as Lereboullet found out 
when he contrasted this species with Perca, for in the latter the nuclei had greater 
transparency and were thus less readily seen (No. 93, p. 513). In this species (Esox) 
Kupffer followed the division of the primary nucleus, and watched the first furrow pass 
down between the two newly formed nuclei ^No. 87, p. 207). 

* Orcjanolocjische Studien, Breslau, 1873-4. 


Around the cleaving nucleus a radial disposition of granules is seen,* the centres of 
the radii being the nuclear apices, for the nucleus itself becomes biconical and shows 
longitudinal striae prior to the division which soon takes place across its middle or shorter 
axis, this transverse separation being followed by the division of the surrounding blas- 
tomere. The process indeed accords perfectly with Balfour's account of the Elas- 
mobranch ovum (No. 15, pp. 394-5). Occasionally a blastomere is seen to contain 
two distinct nuclei, illustrating indeed the stage of the process figured in PL II. fig. 2, 
a stage which Lereboullet also clearly observed, for he says — " Dans un de ces lobes j'ai 
trouve une cellule qui avait deux noyeaux distincts rapproches l'un de l'autre " 
(No. 93, p. 484), and Balfour similarly speaks of a double nuclear condition (No. 11, 
p. 396). Though usually very distinct and centrally situated, the nucleus sometimes 
becomes diaphanous, and appears to be absent. Such an enuclear condition is hardly 
possible, though Professor Kay Lankester, it is true, speaking of the blastoderm of 
Cephalopods, says — " I have most fully satisfied myself that temporarily many of the 
segmentation-products are devoid of nucleus" (No. 90, p. 39); and Lereboullet, when 
noting the fact that all through cleavage each blastomere contains a nuclear body, adds 
that "often it may be absent" (No. 93, p. 484); while Bambeke could find no trace 
of nuclei in Leuciscus rutilus, but accounts for it by the similar refrangibility of the 
nucleus and the matrix in which it is situated (No. 20a). This disappearance of the nuclei 
is not an uncommon phenomenon in cell-division. Very often (PI. II. fig. 1) a body 
apparently of the nature of a clear vesicle occupies the place of the deeply-stained nucleus 
in sections, or such a vesicle occurs only partly occupied by a nuclear remnant (PI. II. 
fig. 1). These unstained bodies were noticed by Balfour, and he felt uncertain whether 
they were nuclei imperfectly stained, or nuclei in course of being formed (No. 11, p. 395). 
In the living egg the phenomena of segmentation are followed without much difficulty, 
especially in pelagic forms. The two primary cleavage-planes are seen to cut each other 
at right angles ; but the third cleft is parallel to the second (PL X. fig. 4). On the 
completion of the third cleft the blastoderm consists of six cells, of which the central 
pair are larger than the others. At this stage the blastoderm is rudely rectangular, an 
outline altered by the next cleft, which passes once more parallel to the second and third 
clefts, through the large central cells (PL XIV. fig. 8). The size of the blasto- 
meres is far from uniform after the 8-cell stage. The 16-cell stage is completed by a 
separate furrow traversing each cell and bisecting it, so that the total number of 
blastomeres is thus doubled at about the fourth or, it may be, the sixth hour after 
fertilisation. It would appear that in the Teleostean ovum, as also in the fowl and 
Selachian, the two primary furrows alone are really regular, the succeeding furrows being 
in varying degree irregular, so that the blastomeres are not seen to increase with the 

* Oellacher observed the concentration of yolk-spherules round one or two centres in the segmentation-spheres, 
but this is not the phenomenon he described, though Balfour understood Oellacher to refer to the behaviour of the 
ordinary nuclei during segmentation. Ryder also speaks of numerous fine granules aggregated round two centres in 
the first cleavage-stage. 


regularity of geometrical progression. The size of the blastomeres is likewise far from 
uniform after the 8-cell stage, and in the 14- to 16-cell stage especially, they vary 
very much in size and shape, the outer being large and somewhat rectangular, while those 
more central are smaller and ellipsoidal. This distinction between the more external and 
the inner cells Balfour noted in Elasmobranchs (No. 11, p. 392), and compared 
it to the horizontal furrow which separates the smaller pigmented spheres from the 
larger spheres of the vegetal pole in Rana (cf. figs. 3, 4, and 5, pi. xv. No. 11, and our 
PI. IX. fig. 8). The form of the disc varies, changing from the circular outline of the 
early blastodisc (PI. XXII. fig. l) to a more or less regular quadrate figure (PI. X. fig. 9), 
and reassuming the circular form when the multicelled stage (morula) is reached (PI. II. 
fig. 13, a). The first furrow parallel to the base of the disc passes across the median 
horizontal plane at about the 50-cell stage (PL II. fig. 14), and the subsequent cleavage 
becomes very complicated. Owing to the increasing pressure of adjacent cells, the 
rounded form of each cell (PL X. fig. 10) becomes altered, and the polygonal shape is 
assumed (PL II. fig. 19). The size of the blastomeres shows much variability, though 
the variation is now within narrower limits. In profile the disc up to this stage has 
maintained the plano-convex outline, which is often retained until the 180-cell 
stage or later (PL X. fig. 10) ; but when the cells are so subdivided as to appear 
almost of one size, a marked bi-convexity is assumed, and upon the yolk a depres- 
sion is formed in which the blastoderm rests (PL II. fig. 2), as it does permanently 
in Salmonoids (Lereboullet, No. 93, p. 485 ; Oellacher, Klein) ; but later it spreads 
out in Gadoids and other forms, and appears as a flattened plaque in which several 
layers of similar cells can be distinguished (PL II. figs. 3 and 15, e). There is 
no marked difference in the cells of the various strata, and the blastodermic layers 
are not readily distinguished, as they are in Elasmobranchs.* Balfour and other in- 
vestigators have made allusion to this similarity in the size and contour of the cells 
of the Teleostean blastoderm {vide Balfour, No. 11, p.. 551 ; and Lereboullet, 
op. cit.). 

It is true, as already pointed out, that in very early cleavage the marginal cells are 
distinguished from the inner cells by a marked difference in size (PL IX. fig. 8); 
nor is the distinction lost with the appearance of the horizontal furrows, though it 
cannot be due, as is undoubtedly the case in Elasmobranchs and Amphibia, to the 
greater proportion of yolk-matter present in the outer germinal protoplasm, for there 
does not appear to be any conspicuous difference in their physical character. 

In the Elasmobranch blastoderm of about one hundred cells, the ectoderm is readily 
distinguished from the endoderm or " lower layer " cells by their smaller size, and marked 
columnar character. Rieneck t observes that the upper cells of the germ give rise to 
a two-layered sensory lamina (or leaf), and that some of the lower cells fall to the bottom 

* Ryder, however, speaks definitely of three layers in the multicelled stage of the Teleostean germ ; hut this does 
not agree with other descriptions hy the same author. 

t Archivf. Mikr. Anat., vol. v., 1869. : 


of the cavity (germinal cavity). Soon after also the larger cells fall off, and we now get 
complete analogy with the Amphibian egg, viz., above the cavity the sensory layer 
composed of smaller cells, and below the large cells for the body of the embryo.* This is 
not the case, however, in osseous fishes, for on the completion of segmentation, an 
epiblastic layer can barely be distinguished: it is not by any means well marked.! 

Germinal Cavity. — With the completion of segmentation the blastoderm undergoes 
a change of the most striking character. It lengthens out (PI. II. fig. 17) and soon 
becomes elevated from the yolk, so that a chamber gc. (PI. II. fig. 15, a-d), not 
coincident with the centre of the disc, is formed between its under surface and the 
vitellus (y) below.| Hitherto the whole of the inferior face of the blastoderm has rested 
immediately upon the yolk (y) (see PI. II. figs. 1-3) or rather upon a portion of the 
yolk-cortex ; but now the inner surface being raised it rests only by its periphery, and 
the eccentrically situated cavity intervenes between it and the vitelline mass. In 
Trigla gurnardus the sub-blastodermic cavity is plainly visible on the second day, 
when the germ covers barely a third of the surface of the yolk. 

A cavity has been observed in some Teleostean ova at a much earlier stage ; but it is 
probably a precocious dehiscence and of minor significance. Such a cavity in the 
gurnard may be formed even before the first cleavage is accomplished, and is probably 
due to the cleavage-process, as we find to be the case in Amphioxus at the 4 -cell 
stage. Agassiz and Whitman found a similar cavity in Ctenolabrus at the 16-cell 
stage, while His describes one at the 8-cell stage.§ Such cavities, of a transitory 
nature, have been noticed in very many ova; in Acipenser sturio, for example, at 
the 6- to 8-cell stage, according to Kowalewsky, Owsjannikow, and Wagner ; 
while Kauber saw it in the Avian ovum at the 4-cell stage (No. 132, p. 6). The last 
named observer distinctly affirms that the early cavity he saw is not the homologue of 
the later embryonic chamber, generally distinguished as the " Keimhohle ; "|| and as this 
is a point of no little importance, it is desirable to dwell upon the distinction here 
implied. The very existence of a cavity, either " segmentation " or " germinal," has 
been denied by some investigators. It has been pronounced by Donitz amongst others 
(No. 52, p. 600) to be merely an artificial product ; and Ktjpffer suggests something of 
the same kind, though unwilling to lay stress upon his results, which were negative 
(No. 87, pp. 214-16). That the somewhat complex methods now adopted in 
laboratory work are calculated to produce occasionally artificial changes in embryonic 

* Rieneck also considered that the embryo originated in one point of the peripheral thickening which occurs at 
the point of contact between the yolk and the germ. 

t Goette affirms that there is no distinct differentiation of any of the germinal layers in the multicelled condition 
of the disc if we except the outer " epithelial " (Archivf. Mikr. Anat., iv., 1868). 

J Rieneck, op. cit., observed the central part of the germ lifted off its underlying part. 

§ It is this cleavage-cavity which Ryder considers as probably homologous with the cavity of the false amnion 
(Amer. Nat., xix., 1885, and Jour. Roy. Micr. Soc, Feb. 1886, p. 45). 

| This later cavity Balfour, in common with most observers, names the segmentation-cavity, though he says it 
is not a well-defined chamber, and remarks that "it may even be doubted whether a true segmentation cavity . . . . 
is present." 


structures is very probable ; but the recognition of a cavity in the Teleostean blastoderm 
has been so general that it cannot be placed in such a category. 

We speak of it as a " germinal cavity," and do so advisedly, for it is not " the cavity 
of Von Baer," better known as the blastoccel or segmentation-cavity. This latter, which 
exists in all segmented germs forming a blastosphere, as in Cylostomes and Amphioxus, 
is, we believe, never formed in such pelagic ova as are referred to here, nor indeed has it 
been clearly recognised in any other Teleostean ova, with the exception of Leuciscus 
rutilus. In this last named ovum Van Bambeke fully describes a true " segmentation 
cavity," though his results are not in accordance with those of embryologists generally. 
Van Bambeke himself doubts the existence of his cavity in the germ of the Salmonoids 
and carps, though closely allied to the form he investigated, and declares it to be 
homologous with the chamber in the ovum of Petromyzon, Acipenser, the Selachians, and 
Amphibians. It is true he quotes Lereboullet in support of his view, and the latter 
undoubtedly does speak of the germ at the close of segmentation as having " un aspect 
granuleux et la forme d'une sphere aplatie qui repose sur le vitellus " (No. 93, p. 503) ; 
but neither his fig. 27, pi. i. nor fig. 3, pi. iii. necessarily imply Bambeke's results, 
nor exclude the existence of the germinal cavity which most authors have seen. The 
segmentation-cavity of Bambeke, the homologue of Balfour's cavity (No. 13, pi. xxi. 
fig. 1, sc), arises as a space in the midst of the blastodermic mass (No. 20a), at 
what period he cannot say, though his figure would indicate an early stage, 
probably when the blastoderm covers a quadrant, that is at the same time as the 
" germinal " cavity, which it also resembles in its non-central position, for it is slightly 
eccentric in position, and in front of the embryonic area proper. It is surrounded by 
blastomeres — the roof, walls, and floor being composed of cells produced by the 
segmentation of the disc. The germ, in which it originates, is essentially a blasto- 
sphere, for though the floor-cells largely disappear, so that the yolk may seem 
partially to form the floor, there is probably never a stage, as Balfour is careful to note 
(No. 11, p. 519), " in which the floor of the cavity is without cells." Balfour, it is true, 
regards the Teleostean germinal cavity as homologous with the segmentation-cavity 
(cavity of Von Baer) in Elasmobranchs and Amphibians (No. 10, i. p. 70) ; but the 
subsequent fate of each of these cavities tells against this homology, for the former is 
persistent, whereas the latter chamber is transitory. If the Teleostean germ after 
segmentation be a morula, which flattens out, and becomes lifted up, and separated by a 
chamber from the appended trophic mass,* resembling in a remarkable manner the 
condition in certain Urochordates (e.g., the cauducichordate Pyrosoma), in which no 
centrally placed segmentation-cavity occurs (vide Huxley, No. 73, and Kowalewsky, 
No. 86, p. 609), then the presence of such a cavity, and the occurrence of a blastospherical 
stage in Teleosteans, must be regarded as problematical. 

* That the blastoderm is actually raised up seems to be demonstrated by the fact that separation may for some time 
be incomplete, connecting strands of protoplasm being frequently distinguishable in the living ovum and in sections 
(PI. II. fig. 15, c), and Ryder is probably in error when he supposes the cavity to arise as a direct result of cleavage 
(No. 141, p. 492). 


Balfour at one time held the view that the floor of the cavity in Selachians was 
not truly blastodermic, the floor-cells arising as concretions around yolk-nuclei at the 
base of the disc (No. 11), and such a cavity would be a germinal, not a segmen- 
tation-cavity like Van Bambeke's ; but later, Balfour relinquished this view, a com- 
plete floor being established, he states (No. 11, p. 43), by the growth inward of lower 
layer cells along with cells formed in the periblast. The cells which Oellacher 
describes on the floor of the " Keimhohle," he says fall from the roof of the cavity, sink 
into the yolk, and multiply (Zeitsch.f. w. Zool., xxiii. pp. 12, 13). The real nature of the 
blastodermic vesicle of Lereboullet is by no means clear, for though Bambeke regards 
Lereboullet's cavity as no other than his own, yet it must be remembered that 
Lereboullet's mucous layer is not necessarily a blastodermic layer in the strict sense ; 
and Van Bambeke himself admits this possibility when he points out the likeness 
of this layer with his intermediary layer (No. 20a, p. 4), a point E. van Beneden 
also insists upon. That Lereboullet himself regarded his " feuillet muqueux " or 
" vegetatif " as extra embryonic, is clear from his denying that it is formed of blastomeres 
— " in fishes and Crustacea (the crayfish) the mucous layer," he says, " is not of the same 
origin as the serous layer" (No. 95, p. 14), the one being the true or animal blastoderm, 
and the other the nutritive blastoderm.* It is not necessary here to decide the real 
nature of the mucous layer, whether it be truly hypoblastic, or hypoblast and mesoblast, 
or neither ; it is sufficient to note that the floor of the cavity, according to Lereboullet, 
has a different origin from the roof, and is not composed of cleavage-products, so that his 
cavity would not seem to be a segmentation cavity at all, and though he considered 
himself justified in stating that the blastoderm is " creuse et forme uneveritable vesicule 
.... dont les parois sont plus ou moins rapproches l'une de l'autre" (No. 93, p. 487), 
yet it must not be regarded as the segmentation-chamber of a blastosphere, but the 
germinal-cavity underlying a morula. If Oellacher be right, that only cells resulting 
from cleavage form the blastoderm, then a cavity, if not floored by such cells, is not 
a segmentation-chamber according to the accepted view regarding that cavity. The 
nature of the floor of any cavity appearing in an early blastoderm is all important, while the 
nature of the roof is not so, being, indeed, subject to variation in very closely allied forms 
like Rana and Triton, one layer of cells forming the roof in the latter (No. 147, p. 453), 
whereas in Rana the roof is two or more cells thick. The lamprey has a multicelled 
roof, which thins out to a single layer, as Shipley has found, in agreement with 
Calberla, and as opposed to M. Schultze ; whereas in Elasmobranchs, as also in 
Ganoids (Acipenser), the ectodermic roof is thickened by endodermic cells which creep up 
the walls of the cavity and pass along the roof. The roof of the germinal cavity in Teleos- 
teans is formed by the whole of that portion of the blastoderm which is raised to form it 
(PI. II. fig. 15&, odm). It therefore includes epiblast (or ectoderm) and lower layer or 

* That Lereboullet's upper layer cannot be the epiblast, and his second layer the entoderm or " lower layer cells," 
is shown by the fact that he speaks of the lower as a single layer (No. 93, p. 492), and the upper as of many regular 
layers of smaller cells, so that our interpretation holds best. 


enclodermic cells. When at its maximum it is a slightly flattened dome-like cavity 
(PI. II. fig. 156, gc) ; but with the extension of the blastoderm its roof is depressed, 
and it thus appears subsequently as a mere fissure. Now Lereboullet figures his 
cavity as a narrow fissure extending almost from margin to margin of the blastoderm ; 
whereas Bambeke's is a compact, but loftier and more spacious chamber.* It is 
noteworthy that Bambeke was struck by this dissimilarity, and after examining the 
segmentation-cavity in the roach was prompted to seek for a germinal cavity underneath 
the blastoderm, and found one, as he indicates in his figs. 4 and 6 (vide No. 20a) ; 
but he adds that " a comparative examination of preparations forces me to regard it as a 
simple accident and artificial, for the prominences and depressions of roof and floor 
coincide." There is much reason to suppose, therefore, from the shape and nature of 
the floor, that Lereboullet's cavity is not a segmentation-cavity, such as Bambeke 
supposes, and, if this be so, then Lereboullet likewise discovered this flattened 
germinal cavity, as E. van Beneoen says (No. 25, p. 47), though this author is wrong 
in according the discovery also to Van Bambeke. If, on the other hand, Lereboullet's 
be really Von Baer's (and Van Bambeke's) cavity, then H. Rathke first signalised the 
germinal cavity in Zoarces ; and he was followed by Stricker. It is therefore not correct 
to speak of a cavity of Lereboullet with Van Beneden but rather of a (sub-blastodermic) 
germinal cavity, which is persistent through all embryonic life, as distinct from the (intra- 
blastodermic) segmentation-cavity which wholly disappears.t 

What then is the significance of the germinal cavity thus distinguished ? By the 
fact that its floor is formed of yolk, or rather the protoplasmic cortical film (or inter- 
mediary layer), and that it is roofed over by endoderm (lower layer) and epiblast-cells, it 
is comparable to the " Keimhohle " in the fowl's ovum.| At a later stage the hypoblast- 
cells which intrude from the periphery to form the blastodermic rim (br) and shield 
(PL II. figs. 15, Or- e, and 17) do not pass across the floor of the cavity, but creep 
up the sides and partially arch it over, forming in fact a gastrula which would open ex- 
ternally by the blastopore, were not this aperture plugged up by the mass of yolk (really 
Ecker's plug), which is so large that the invaginated lip is compelled to pass round, and 
epibolically envelop it. The germinal cavity, arched over as it is by the thick blastodermic 
roof, bdm (PI. II. fig. 15, a-e), is never truly open in the sense indicated; but potentially it 
is so, the removal of the concentrated trophic matter (y) which does not segment would leave 
the blastoderm a simple gastrula — indeed, as Ryder remarks in regard to Alosa, that 
" the yelk might be removed at any stage without taking away any essential part of the 
embryo except the floor of the cavity " (No. 141, p. 569). Van Bambeke does not hesitate 
to regard his chamber as a gastrula-cavity, and finds in it therefore great phylogenetic 

* A glance at Lereboullet's figure (No. 93, pi. iii. fig. 3) and Bambeke's (No. 20a) sufficiently shows this. 

t See a paper " On the Significance of the Yolk in the Eggs of Osseous Fishes," by E. E. Prince, Ann. Nat. Hist., 
July 1887. 

X It is interesting to observe that, with the appearance of the germinal cavity, the thick periblast-floor in some forms 
becomes thinner. The Keimhohle or germinal cavity is often called the segmentation-cavity in the fowl's ovum. 


significance; but Oellacher, Donitz, Ryder, and others agree that it is merely an arti- 
ficial product, and due to the action of reagents. It is difficult to accept the latter view, 
after the careful observations of Van Bambeke, who admits that in the trout and carp 
it is absent, as seems to be also the case in a large number of Teleosteans at St Andrews ; 
yet since a cavity of this nature, remarkable for its deep situation and transient nature, 
has been seen in other blastoderms (e.g., Aves and Ganoids), it may justifiably be 
regarded as a normal structure, and perhaps due rather to the exigencies of the cleavage- 
process than to ancestral causes. If, as Whitman holds (No. 159, p. 296), "the case of 
Ascidia (Kowalewsky), of Sycandra (Schultze), of Anodonta and Unio (Flemming), of 
Clepsine and Euaxes, and numerous cases like the latter, show that the blastocoel arises 
by the cells being pushed asunder in the process of cleavage," then the segmentation- 
cavity when it is present can have no profound ancestral meaning, such as Van Bam- 
beke urges ; but is of interest merely in connection with modifications in the ovum, by 
which the area embraced in segmentation is greatly reduced. This reduction impli- 
cates a mechanical difficulty, resulting in the formation of a chamber, which is appro- 
priately named a segmentation-cavity or blastocoel. Probably every instance of a 
blastocoel may be explained in this manner, and it may thus co-exist along with the 
germinal cavity. The former, it is generally admitted, becomes obliterated, whereas the 
latter persists, and must be regarded as the remnant of the primitive enteron. Its 
persistence in the embryo is of importance, for it is an essential point in the gastrula 
that "it should directly or indirectly give rise to the archenteron " (No. 10, p. 457). 
That in forms so various as G alius, Rana, Acipenser (No. 82), and Balanoglossus 
the segmentation-cavity is transient, and has no relation to the blastopore, is proof 
that it cannot be regarded as enteric, for the archenteron has always relation to the 
blastopore. In speaking of the cavity in the Teleostean ovum as germinal, we merely 
do so to distinguish it from the segmentation-cavity (blastocoel), which is wholly 
another structure, though the name does not necessarily imply any ulterior meaning. 
Nor is this course discordant with the conclusions of Teleostean embryologists in general ; 
for Oellacher distinctly affirms that the germinal cavity produced by the lifting up of 
the germinal mass is the sole cavity observed by him in Sahno fario, and he failed to 
find a central segmentation-cavity, as was the case also with Van Bambeke in the ova 
of this species, and of Cyprinus; and Klein, though he speaks of a segmentation- 
cavity, formed by the lifting up of the blastoderm, really means the germinal cavity 
(No. 79, p. 197, and pi. xvii. figs. 11 and 12), this latter cavity being also recognised 
by Rieneck (No. 137, p. 356), Gotte, Henneguy, Owsjannikow, and Weil. Janosik 
observed a cavity in the germ, and an earlier one between the yolk and the lower layer 
cells, and he termed the former " segmentation-cavity."* It is not a little curious that 
Ryder, while holding that the germinal disc of Teleosteans is equivalent to the entire 
Amphibian ovum, yet regards the cavity outside the disc (germinal cavity) in the former 
as homologous with the deeply placed chamber (segmentation-cavity) in Rana and the 

* Archivf. Mikr. Anat, vol. xxiv. 
VOL. XXXV. PART III. (NO. 19). 5 Y 


Elasmobranchs, and somewhat inconsistently maintains that in Teleosts the origin of the 
cavity is directly due to cleavage ; whereas, on phylogenetic grounds, it must arise in 
connection with the peripheral invagination and the formation of the blastopore. If, as 
Ryder holds (No. 141, p. 492), the Teleostean germ is equivalent to the whole ovum of 
Rana, then we must look for a segmentation-cavity deeply placed in the former blasto- 
derm, a fact which Bambeke, as we have seen, considers established for Leuciscus. 
Ryder, too, adopts a questionable view of the germinal cavity, when he says that it is 
" simply a space filled with fluid, which facilitates the gliding of the blastoderm over the 
yelk during growth," and constituting the fissure between the outer (embryonic) layer 
and the inner envelope of the yolk, and further as the representative of a " primal 
nutritive space," a lymph-cavity. He also considers that the body-cavity is continuous 
with the segmentation-cavity, and maintains that it does not disappear in Gaclus 
morrhua, Cyhium, Coregonus, and Alosa. 

While there are many points, therefore, which support the view that the segmentation- 
and germinal cavities are not one, but may indeed co-exist, or may appear successively 
in the same ovum, there is a possibility that the difference between the deep-seated cavity, 
seen, for example, in Elasmobranchs, and the sub-blastodermic chamber in Teleosts, may, with 
extension of our knowledge of the early blastoderm in the latter, disappear, and this would 
be so if it could be shown that the germinal cavity arises, not by the lifting up of the 
disc, but by intracellular dehiscence, and the disappearance of the lower (separate) 
stratum, i.e., the blastomeric floor.* At present the germinal cavity must be distinguished 
as such, the characteristic features being its situation superficial to the yolk, the absence 
of blastoderm-cells separating it from the granular yolk-cortex, and its persistence even 
into the later embryonic period. Other minor features justify us in emphasising the 
distinction of this cavity from the blastoccel or segmentation-cavity proper. 

VI. Periblast or Nuclear Zone. 

From the way in which the protoplasm of the ovum collects at the animal pole, it 
is readily seen that the continuity of the disc and the cortical protoplasm beyond does 
not cease for some time, and that even when the blastoderm by cleavage has become 
defined in the form of a cellular prominence, its connection with the unsegmented 
protoplasm external to it is most intimate. The process of superficial transference still 
proceeds after cleavage has commenced, t 

* The fact that during a considerable interval the segmentation-cavity in Elasmobranchs is greatly deficient in its 
cellular floor, and the yolk limits it below (No. 11, p. 518), is interesting, though Balfour doubts if ever the yolk alone 
forms the floor (p. 519). Gotte's observations would demonstrate the existence of such a floor of cells in the Tele- 
osteans, though it is always incomplete. 

t Granular yolk is also transferred in the Elasmobranch, both Oellacher and Balfour agreeing that yolk is 
assimilated by the germinal area during segmentation. The cessation of the transference and of the yolk cell-gemma- 
tion accounts in a great measure, according to Balfour, for the comparative distinctness of the disc and the yolk at the 
end of segmentation. 


So long as the yolk-ball can be distinguished even in advanced embryonic stages 
(see PI. VII. figs. 1, 9, &c, cp), it is provided with an envelope of unsegmented proto- 
plasm especially noticeable round the margin of the disc (per, PI. II. fig. 12), and forming 
in the early stages of cleavage a thickened peripheral belt. This envelope is the " feuillet 
vegetatif ou muqueux" of Lereboullet (No. 93, p. 771); the "trophic or glandular layer" 
of Remak (No. 135, p. 342) ; the "parablast" of Klein (No. 79, p. 116) and His (No. 
67); the " Korner-zone" of Kupffer (No. 88, p. 217, fig. 1) ; the " lamina mycogastralis" 
of Haeckel (No. 62); and the yolk-hypoblast of Ryder, (No. 141); but appropriately 
distinguished as the "periblast" by many authors. 

We may speak of the periblast as early as the stage of first cleavage, the two primary 
blastomeres constituting the germ proper as distinct from the protoplasmic layer beyond.* 
The distinction, it is true, is more apparent than real, for the protoplasm at the margin 
of the disc is in a state of continual transition, passing into the germ probably during the 
whole cleavage-process, the disc being indeed only a thickened portion of the proto- 
plasmic cortex of the egg, — " a lenticular enlargement of the Rindenschicht," as 
Oellacher expresses it (No. 113). In thus regarding the periblast as an aggregation of 
protoplasm which lies outside the germ proper, because it has reached the animal pole too 
late to enter the disc and take part in cleavage, we adopt a theory of its origin which has 
been questioned by some observers, notably by Agassiz and Whitman (No. 2). These 
observers suggest that the periblast is really a product of the blastoderm ; that, instead of 
being, as we have expressed it, too late to enter the disc, it has already formed part of 
that structure, and has been protruded as a germinal outgrowth all round the margin 
during segmentation. Van Bambeke, as if by anticipation, expressly opposes such a 
view, and says — " It cannot originate from the disc ; it is coarsely granular, like the cortex 
(le manteau protoplasmique);" but he goes on to state that the cortex wholly disappears 
when the intermediary layer is formed, whereas the cortex persists very much longer, 
though so thin that, as he says, " it is difficult to detect " (No. 20a). 

It is not easy to controvert a view which denies the independent origin of the 
periblast, for its apparent extension outwards from the margin of the disc and the 
continuity of both would seem to favour it. But, if it be correct, then at one stage all 
the superficial protoplasm of the ovum must be collected into the germ-mass ; and no 
such complete segregation has been observed — a stratum of cortical protoplasm continuous 
with the germ is always discernible up to the stage when the periblast can be distinctly 
recognised as a nucleated layer. Its extension beneath the disc is implied in the view here 
adopted, for the superficial protoplasm collects beneath the disc as elsewhere, and this 
can be observed by the behaviour of the oleaginous sphere in such an ovum as that of T. 
gurnardus, inasmuch as it passes along beneath the floor of the germinal cavity evidently 
prevented by the layer of continuous protoplasm from entering the chamber. Van 
Bambeke, it is true, questions this latter point, saying that at one time no trace of a 

* Kingslet and Conn, in mentioning that complete furrows in segmentation pass downward to the vitelline 
globe, except the intermediary layer and peripheral cushion of Van Bambeke. We agree with this view. 


central lamella can be seen, and that it is " most probably formed by extension under 
the disc from the bevelled ring outside." Like Agassiz and Whitman, Lereboullet 
holds that this layer is formed later than the disc, observing that, " at the close of seg- 
mentation, no trace of the mucous layer is seen, though dispersed vitelline globules are 
visible out of which this layer is formed" (No. 93, p. 495). 

Three theories of the origin of the periblast are thus held — (l) that it is simply a 
separation, a superficial segregation of protoplasm interfused in the yolk, and reaching 
the animal pole too late to enter the disc ; (2) that it does form part of the disc, but 
afterwards issues from it all round the margin, extending as an extra-germinal layer ; (3) 
that it is not a mechanical transference, but an actual transformation of yolk-particles. 
The second and third views, just stated, involve processes less simple than the first, and 
if a process of simple transference, the segregation of interfused germinal matter, suffice, 
it is needless to resort to any explanation more complex. The superficial segregation of 
protoplasm implies that a sub-blastodermic stratum is never wanting, and that, from the 
first, the blastomeres "do not rest" (in E. van Beneden's words) "immediately on the 
vitellus ; they are separated from it by a layer of substance which is finely granular " 
(No. 25, pp. 44, 45). 

For some time the periblast remains homogeneous, devoid of nuclei, and not separable 
from the yolk-cortex beyond, save by its slightly greater thickness (per., PI. II. fig. 14), 
and by the occurrence of scattered granules in it, which are distinctly seen at the end of 
the first day in G. morrhua. Further, the occasional presence of protoplasmic filaments 
over the area of the periblast seems to indicate its tenacious character (PI. II. fig. 7). 
It forms in some ova, as Lereboullet and E. van Beneden noted, a considerable 
thickening below the centre of the germ. This thickened central lamella disappears 
later, and it is doubtful whether in many species it is ever present. The peripheral 
thickening is usually well marked * as a prism-shaped ring (per, PI. II. figs. 1-3), which 
is triangular in cross-section, the disc resting upon one side, the lowest side being in 
contact with the yolk, while the third is external and free. When segmentation is 
far advanced and the biconvex form has been assumed, large nuclei begin to appear in 
close proximity to the margin of the germ (PI. IX. fig. 1 0, n). Though irregularly disposed, 
two or three rows may be distinguished (PI. IX. figs. 9 and 10, n; and PI. II. fig. 4a), 
and they rapidly extend outwards over a variable area, which is known as the nuclear 
zone. The nuclei are large clear vesicles, having a slightly pinkish hue in certain lights 
(transmitted), well-defined and rounded in form, often slightly elliptical, and sho wing- 
in some cases granules or nucleoli (PI. II. figs. 6, 8, n ; and PI. IX. fig. 9). At first 
they are crowded together, but as they extend towards the equator they show a 
tendency to a regularity of disposition which is very remarkable when they are five 
or six deep. Kupffer describes these bodies in certain species of Gastrosteus as 
larger than the nuclei of the germ, separated by regular intervals three times the 
diameter of each nucleus, and arranged in rows duly alternating, the row nearest to the 
* Lereboullet descants upon its unusual thickness in the trout (No. 95, p. 14). 


disc being the first to appear, the rest following in succession (No. 88, p. 217). At the 
12th hour, in the gurnard, the nuclear zone forms a conspicuous spotted belt round the 
disc, and the yolk in certain views seems to be distinctly pitted by them (PI. II. fig. 5). 
A little later they are less distinct. When the blastoderm has extended over one-third 
of the yolk-surface, traces of the nuclei are still to be seen (PI. XIV. fig. 7, np). Thus, 
at the 25th hour, in the gurnard the blastoderm is surrounded by a continuous belt 
of protoplasm, beyond which few or no granules exist. Those previously seen have been 
overlapped by it, but are visible underneath towards the rim. In the surrounding 
protoplasm no large nuclei appear, and only a few of the granules of the previous stage. 
Often, during the early period of the nuclear zone, the nuclei appear in groups as if 
multiplying by division, this being well marked in the ova of Gadus morrhua; but on 
the second day the nuclei are invisible, and only a granular ring surrounds the disc. 

How do these nuclei arise 1 Three possible geneses are suggested, — they may be 
derived from the nuclei of the blastoderm, as Schultze, Oellacher, Whitman, and 
Wenckebach (No. 158)* hold ; or they originate directly or indirectly from a primary yolk- 
nucleus (Hoffman, E. van Beneden) ; or lastly, they may be endogenously formed as 
independent segregations of active protoplasmic particles (Kupffer), either from the 
marginal cells, or from the cells which fall from the lower surface of the " segmenta- 
tion-cavity," or rather germinal cavity, and which fuse with the periblast. Wenckebach 
asserts that no nuclei or cells arise either in the periblast or in the yolk, and that the 
nuclei of the periblast, after their separation from the blastoderm, degenerate and take no 
part in the formation of the embryo.t The appearance of the extra-embryonic nuclei later 
than the nuclei of the germ — further, their first manifestation close to the margin, and 
their increase centrifugally from the blastoderm, point, it cannot be denied, to a blasto- 
dermic origin. Their derivation from an original single yolk-nucleus has not been 
demonstrated by any observations, nor does it appear to be supported by the manner in 
which the nuclei become visible, though it accords best with the theory that the multi- 
nucleate condition is less primitive than, and derived from, the uninucleate. This con- 
tention Butschli has devised, and he adduces the case of certain Infusorians in which not 
only is the multinucleate condition prior, but actually gives rise to the uninucleate con- 
dition — many nuclei coalescing before nuclear cleavage takes place (No. 36, pp. 212-13). 
It must be observed, on the other hand, that Englemann (No. 54, pp. 576-7) and Zeller 
(No. 161, p. 360) have shown that in Opalina the multinucleate is unmistakably derived 
from a primary uninucleate condition. The existence of a primary yolk-nucleus in 
Teleosteans still remains to be demonstrated. If, by segmentation of this nucleus, the 
periblast-nuclei are produced, appearances in the living ovum afford little evidence of 
it ; but if the nucleus dissipates, and later, becomes aggregated again at numerous 
superficial centres, then this view is not without support. 

Kupffer, Klein, and other authors regard the nuclei we are considering as free 

* Ryder recently adheres to this view (U. S. Com. Report for 1885 (1887), p. 490). 
t No. 158, and Jour. Roy. Micr. Soc, Feb. 1887, p. 43. 


nuclei, originating as independent segregations of active protoplasm, like the nuclei which 
arise endogenously in the Molluscan ovum, as Professor Ray Lankester was the first to 
recognise. In Crustacean ova such nuclei have long been known, though in Oniscus 
it is noteworthy that Bobretzsky affirms their blastodermic origin and subsequent 
migration ; but this view is not generally accepted. Weissman, too, imagined that in 
the ova of Dipterous insects such structures arise de novo, and without genetic relation 
to nuclei already existing ; but later researches lend little countenance to this opinion, 
and Weissman has abandoned his contention. Kowalewsky has described in the yolk- 
matrix of the Annelidan ovum scattered nuclei, endogenously formed and afterwards 
collecting superficially, especially beneath the blastoderm ; they are at first few in num- 
ber, but show rapid increase, and are especially abundant about the time of exclusion. 
He regards the nuclei of the " intermediary layer " in the Teleosteans as originating from 
those of the entoblastic (yolk-) cells. The appearance of free nuclei in the region outside 
the embryonic area in the chick, as described by Rauber (No. 133, p. 570), is a further 
instance of such extra-embryonic nuclear bodies, and the nuclei in the Teleostean 
periblast may have a like origin.* The fact that they differ in shape from the 
spherical nuclei of the disc — being generally more or less elliptical, and often of 
larger size (PL II. fig. 6, n) — points to a non-blastodermic origin. Kupffer speaks of 
their differentiation, and of delicate contours which appear round them resembling hexa- 
gonal figures, in Clupea (No. 87, p. 205). Lereboullet observes that they are large and 
granular in JEsox, and along with the matrix in which they lie, they "come from an- 
other source " than the protoplasm and nuclei of the disc (No. 93, p. 494). Balfour, again, 
comes to the conclusion, while leaving their origin an open question, that there is no 
evidence of their derivation from pre-existing nuclei in the blastoderm (No. 10, p. 109). 

In the living Teleostean ovum it is difficult to watch the actual formation of these 
nuclei ; but Kupffer describes with some detail the appearance in Clupea of clear spots 
of protoplasm which grow from a speck-like particle to a body 5-6 jx in diameter (op. 
cit., p. 201), and E. van BENEDENt is no less decided in affirming that these nuclei 
arise " par voie endogene " simultaneously in the periblast. We have noted that in 
the egg of the cod, towards the end of the first day, the periblast shows only minute 
granules scattered through its translucent protoplasm. The nuclei | are few at first, 
and close to the edge of the disc, as if some of them had escaped by " hernia." At other 
parts of the periblast clear vesicles and minute granules occur. Observations do not 
strongly support the view that the nuclei of the periblast migrate from the archiblast, 
but probably they arise in the periblast itself, and it may be that the activity in the disc 
proper stimulates similar activity in the periblast, just as a limited area of irritation in 

* Ryder regards the " nuclear zone" as homologous with this germinal wall in the chick, and it is certainly note- 
worthy that the nuclei in the latter (the " white yolk nuclei ") are most abundant below the thickened periphery of the 
blastoderm, and become the nuclei of cells which enter the germ. t Belg. Acad. Sc, No. 6, June 1876, p. 1202. 

X Kingsley and Conn {op. cit., p. 199) observed in the dinner the formation of cells round these nuclei on the 
surface of the yolk; but it seems, according to Mr G. Brook, that Mr Kingsley has since altered this view {Trans. Roy. 
Soc. Edin., 1887, p. 224). 


ordinary vertebrate tissues has a tendency to stir up a like condition in surrounding parts. 
So early as the 32-cell stage in T. gurnardus, numerous nuclei, precisely like those after- 
wards present in the periblast, were observed, irregularly scattered beneath the blastoderm. 
Some of these nuclei, which were in close proximity to each other, coalesced and formed 
large irregular structures. 

On one occasion careful focussing brought out beneath the cells of the blastoderm (in 
an ovum of the species just referred to, of which the yolk was about half enveloped) the 
faint outlines of periblastic nuclei, while, in an oblique view of the invaginated rim its 
under surface was somewhat regularly nodulated by the nuclear projections which thus 
protrude into it from below (PI. II. fig. 5). 

The blastoderm of Gastrosteus spinachia at a certain stage shows, scattered through- 
out its extent (PI. II. fig. 9, n), large bright nuclei, often showing many nucleoli. These 
nuclei, as suggested elsewhere (No. 124, p. 493), are probably periblastic, and they 
persist for some time after the closure of the blastopore. 

After their appearance close to the margin of the disc, they extend outwards, while 
at the same time they also pass inwards, and form a nucleated stratum beneath the 
blastoderm. They progress centripetally, and eventually stud the periblast-floor of the 
germinal cavity, and are visible through the roof formed by the translucent blastoderm ; 
but whether they increase by cleavage or spontaneous endogeny is not clear. Balfour 
states that they increase by division (No. 10, p. 109), and nuclei frequently show a 
transverse line coinciding with the short diameter (PI. IX. fig. 10), but the further 
constriction and " direct " division of an example of such nuclei into two daughter-nuclei 
was not made out, # and it is probably true that they arise and multiply precisely like the 
nuclei named "autoplasts" by Professor Lankester in the ovum of Cephalopods — arising 
and multiplying not by cleavage, but originating de novo as independent segregations. t 

The behaviour of the nuclei outside the disc in Teleostei is similar to that in Elasmo- 
branchs, as Balfour clearly states that whatever influence the nucleus may have in 
ordinary cases of cell-division, it may yet undergo precisely similar changes without 
exerting any influence on the surrounding protoplasm. In Elasmobranchs the nuclei of 
the disc are rounded and regular in form, while those in the yolk are irregular in shape, 
and provided with knob-like processes. The cone-like nuclei are only found in the earlier 
stages, and they possess no distinct membrane. 

Oellacher, who refers more especially to the nuclear zone as described by Kupffer, 
says there isno need to resort to free-cell formation, inasmuch as its protoplasm is the 
same as the rest of the archiblast, hence, in each, the segmentation-process is the same. 
Bambeke ingeniously suggests that an endogenously-formed yolk-nucleus may give 
origin to these nuclei, and that the cells of which they are the centres are segmented 
more slowly than the cells of the disc (No. 20a, p. 4) ; but, as previously noted, 

* The failure to observe " direct " division will not, of course, appear strange to those who accept karyokinesis or 
indirect division as the sole process of nuclear multiplication, but all visible forms of division are here included. 

t Lankester is also of opinion that the cells of the perimorula in Gammarus locusta arise as. isolated structures 
like the autoplasts of Cephalopods (No. 92, p. 63). 


neither a primary nor a later endogenously-formed yolk-nucleus has been made out in 
the vitellus of the Teleostei. Upon this vexed question centres the interpretation of 
the trophic part of the ovum. 

That the periblast-nuclei are really autoplastic, would seem to be the conclusion most 
agreeable to the facts of the case,* and if the yolk were ancestrally divided into separate 
nucleated masses or cells, as was most probably the case, then upon the breaking down of 
these yolk-segments, to form the existing syncytium of the Teleostean ovum, the nuclear 
matter would likewise become diffuse. It is possible, therefore, to look upon the peri- 
blastic nuclei as the revival (segregation) of the primary nuclear bodies. The vitellus in 
one species (Temnodon saltator), described by Agassiz and Whitman (No. 2, p. 14), 
still shows the division into large yolk-segments without nuclei, though the segmenta- 
tion is not total, a large central mass remaining uncleft. These large segments are 
much flattened, and appear beneath the marginal periblast, with which, during epiboly, 
they progress round the central yolk-nodule towards the vegetal pole. A similar con- 
dition occurs in the pelagic egg of the sole (PL XXII. fig. l), in which a series of vesicles 
or segments appear under the disc in the lenticular stage, and spread with the blastoderm 
so as to form a superficial layer over the entire yolk. In the extremely pellucid egg of 
the sprat, again, the whole yolk is imperfectly divided into a series of polyhedral masses. 

Even holding to the position that the cell is essentially of a uninuclear character, no 
difficulty is presented by the multinucleate periblast, for each may be regarded as the 
centre of a cell whose outline is undefined. It must be granted also that little difficulty 
is presented to those who regard the yolk as a single cell — if, as Butschli holds, a single 
cell during proliferation may exhibit all the gradations from a uninuclear to a multinuclear 
condition, and from the latter retrogress to the former condition without once forfeiting 
its character as a single cell. On the other hand, the syncytium, as Haeckel conceives 
it, though formed of cells originally separate, and including therefore many nuclei, is 
still a cell. 

There are many appearances in the living ovum which indicate that the periblast 
contributes cells to the blastoderm, such cells being segmented extra-embryonically.t 
This point belongs to a later stage of development, and we can here merely make a 
reference to this segmentation of the periblast in its bearing upon the real significance of 
this layer. 

In an ovum of Gadus ceglefinus, at the close of the first day after fertilisation, the 
nuclear zone was well marked, and the homogeneous protoplasm composing it rose into 
minute prominences or depressed conical papillae, upon each of which a nucleus appeared 
to be seated (PI. II. figs. 4 and 4a, n). This botryoidal appearance was unmistakable, 

* Tt would not be accurate to speak of these nuclei as genuine " autoplasts," for these latter bodies never become 
the centres of cells produced by cleavage. It is essential to the autoplast that the surrounding matrix remains unseg- 

t The growth of the blastoderm by marginal conversion of cells is a phenomenon that continued investigation shows 
to be widespread ; it occurs in many Invertebrates — in Cyclostomes, and, as Balfour and Deighton unmistakably demon- 
strated, in Birds. Vide "Renewed Study of the Germinal Layers of the Chick" {Quart. Jour. Micr. Sci., xxii. p. 177). 


and due, there can be no doubt, to planes of cleavage passing as linear depressions from the 
margin of the disc outwards. No cells could actually be seen to be completely segmented 
and added to the margin of the disc, nor could this be ascertained by study of the living- 
ovum, for such cells transferred into the germ would enter the lowest stratum of the disc, 
and would therefore pass beneath the margin along the basal region — this margino-basal 
portion of the blastoderm being especially unfavourable for study in the living condition. 
There is no evidence against Brook's view, that matter passes into the archibkst in the 
early stages, and thus nourishes it — a view similar to that of Hoffman (No. 68), viz., 
that the nucleated periblast performs the function of provisional blood. 

VII. Embryonic Shield and Rim. 

We have traced the development of the ovum up to the stage which immediately 
precedes the formation of a distinct embryonic trunk, coincident with the radial 
thickening of the blastoderm. No clear differentiation into layers can as yet be 
made out, though the upper stratum is usually distinguished as a layer of ectoderm 
(Oellacher's "hornblatt") or epiblast (PL II. figs. 1-3, 6, and 15, ep) — the cells 
below, which form the main mass of the germ, being endodermal or lower layer 
cells (11). This saucer-shaped blastoderm (PL II. fig. 19), consisting of two germinal 
leaves or layers, arches over the germinal cavity, while peripherally it is in contact 
with the cortical protoplasm of the yolk, chiefly that part of the cortex distinguished 
as periblast. Then commences epiboly, that remarkable process which Rathke, in 
1832 (No. 129), was the first to describe in Teleosteans. The germinal matter which 
originally clothed the vitelline globe as a film, and afterwards becomes segregated 
at the animal pole, is now seen apparently retrogressing, and again encloses the yolk, 
not as a homogeneous envelope, however, but as a segmented cellular blastoderm. 
With the commencement of the process the blastoderm flattens (PL II. fig. 15, hdm), 
and the vertical height of the germinal cavity (gc) is by this depression so much 
reduced as to form a mere fissure, though otherwise its relations remain unaltered. On 
the second or third day, in the Gadoids and other forms here referred to, this flatten- 
ing is clearly shown ; and Lereboullet, who describes it in Esox, says that during the 
first half of the second day the blastodermic vesicle (i.e., the germ) flattens more and 
more, its two opposing walls touch,* and it becomes moulded as a serous envelope round 
that part of the egg which it covers like a watch-glass (No. 93, p. 488). By this process 
of flattening and extension meridionally over the yolk-ball, the germ becomes distinctly 
thinner. This decrease in thickness is especially noticeable, Lereboullet says, in Salmo 
fario as compared with Perca, and epibolic extension in the trout is much less rapid 
than in the latter. Variations, too, occur in pelagic ova, but these are doubtless caused 

* If our interpretation of Lereboullet be correct, it is not accurate to speak of the two layers, viz., the thin germ 
and the periblast, as really touching, though the interspace becomes less and less. 

VOL. XXXV. PART III. (NO. 19). 5 Z 


in a large measure by differences of temperature, light, the condition of the water, and 
other features of the laboratory, though the divergence between a pelagic and a demersal 
ovum in this respect is so marked as not to be fully explained in that way. Thus in 
Gastrosteus a blastoderm, which covered fully one-eighth of the yolk, had embraced in 
twenty-four hours only slightly over a quadrant, while in Pleuronectes it had extended 
over nine-tenths of the yolk-surface. Again, in Gadus ceglejinus, when the temperature of 
the tanks was kept lower, epiboly was as slow as in the case of Gastrosteus under a higher 
temperature. When the germ covers barely a quadrant the margin becomes visibly 
thickened, this being the first indication of the embryonic rim (Kupffer's Keimsaum, 
Oellacher's Keimwulst), which plays so important a part in the formation of the 
embryo (PL II. fig. 17, br). This appearance of the rim Lereboullet connects with 
the thinning out of the germ, and explains it as a process of mechanical transference — 
the central cells passing to the circumference, as indicated by the increased density 
of the latter, which forms "a true pad around the egg" (No. 93, p. 458). The cells 
of the germ undoubtedly become greatly flattened, as we see in PL II. fig. 3, as com- 
pared with fig. 17, when extension has proceeded largely; but such a transmission of 
cells less truly represents the process of peripheral thickening than the inflection of 
old, the reception of new cells described below, and the aggregation of these in a 
marginal band. 

We have referred to epiboly as to all appearance a retrogression,* but it is not really 
so, it is rather a process of invagination such as we find so widespread in the develop- 
ment of animal germs. This process, had the amount of food-yolk present allowed, 
would have resulted in the establishment of an involuted epithelial lining to the gastrula. 
The exaggeration of the trophic mass, which must ancestrally have been even much 
greater, prevents this progress of the ectoderm, and as its extension is not arrested, it 
follows that the yolk-globe is epibolically enveloped. While, as indicated, the germ 
becomes thinner, yet along one radius this decrease is not so great as elsewhere ; 
in other words, the germ, soon after the close of segmentation, shows a thickened 
embryonic radius which never disappears (PL II. figs. 15 and 17). When the 
germinal cavity (gc) is formed, this portion is well marked, as the cavity lies in front 
of it, i.e., eccentrically, and all through development it is thus distinguished by its 
greater thickness, so that Lereboullet cannot be correct in saying that the embryonic 
radius only commences when epiboly is nearly complete (No. 93, pp. 495-6). He failed, 
indeed, to notice in his species any trace of the rim until the blastopore is in its final 
stage, then, he says, a very distinct rim is formed around the " trou vitellaire " of Vogt. 
In the trout, as Oellacher shows (see No. 114, Taf. i. figs. 2-5), this radius is well marked ; 
but in other forms it is less apparent at an early stage, though the (embryonic) radius 
in all Teleostean ova is probably distinguishable from the non-radial portion by its 
greater thickness. In sections through the blastoderm before the equator is reached 
(PL II. fig. 17, and PL IV. fig. 8), the germ consists merely of two layers — ectoderm (ep) 

* Vide E. E. Prince, Annals Nat. Hist., July 1887. 


and entoderm (hy), the cells of both being very much flattened ; but along the embryonic 
axis several layers are present, and the cells are, in the living germ, more rounded and 
fuller than elsewhere. Similar larger cells also occur at the margin (PI. IV. figs, bf, 
and 7), and to the presence of these, as well as their closer arrangement, no less than the 
greater number of cells, is due the thickened appearance of the marginal belt or rim (br). 
It is clear that the blastoderm covers a very large superficial area, when compared with 
its extent at the close of segmentation, and this extension is largely, as we have hinted, 
a process of "flattening out" undergone by the originally rounded or polygonal cells of 
the archiblast. The cells are thus expanded superficially; but doubtless there is also a 
marginal addition of cells — periblastic in origin. 

Beneath the rim and embryonic axis a single layer of cells intervenes, separating the 
germ from the yolk. This layer is, in fact, the third primary layer or hypoblast 
(Darmdrusenblatt), and its mode of origin is a point of great interest. How does it 
arise ? The answer to this question is by no means easy, but the view that it is 
invaginated, i.e., an inflection of the epidermal layer, is grounded upon appearances in 
the living ovum, and prepared sections (PI. II. figs. 15, 17, hy) no less than upon 
phylogenetic considerations. A folding-in of the epiblast is indeed seen at a very early 
stage, but, when the germ has thinned out, this involution is more apparent (PI. II. 
figs. 10, 17), and the centripetal advance of the rim can be readily followed by continuous 
watching, for, starting as a narrow peripheral band very slightly denser than the rest of 
the blastoderm, it advances slowly towards the central point of the animal pole. 
This region, known as the embryonic scutum (Oellacher's Embryonalschild), coincides 
with the embryonic radial thickening, which, as already noticed, is present from a very 
early stage. Lereboullet calls it the " bandelette primitive" or " germe embryonnaire," 
as being in his view the first indication of the embryo (No. 94, p. 255), but this is not 
so, the thickened radius preceding by an interval of many hours the inflection of the 
hypoblast, and being already distinguishable, when the germinal cavity appears. At 
first the scutum is a mere tubercle in the Salmonoids, as Lereboullet says, though 
flatter and more tongue-like in Gadoids, which pushes out from the rim and progresses 
towards the pole opposite to the blastopore. As it advances and extends laterally, it 
brings visibly into prominence the embryonic thickening, which, however, already exists, 
and when the blastoderm covers about one-fifth of the vitellus, this hypoblastic layer 
spreads out as a scutiform film or membrane beneath the embryo. That this process is one 
of true invagination is disputed. Gotte, Henneguy, Cunningham, Kingsley, Conn, and 
others hold that it is so ; whereas Oellacher regards it merely as a delamination, 
a simple differentiation in situ of the deepest layer of the primary entoderm, and this 
view Ryder and others adopt. Kupffer, Van Bambeke, His, Klein, and G. Brook 
regard the sub-blastodermic protoplasm or periblast as the source of this layer. 
Lereboullet speaks of it as a vegetation or proliferation (No. 94, p. 253), though he also 
seems to resort to a kind of mechanical transference of cells (No. 93, p. 488). We know 
that in Elasmobranchs this layer is formed partly by conversion of lower layer cells 


in situ, and partly by invagination ; in Cyclostomes and Amphibians it is in all likelihood 
invagination purely, and the prevailing view, that Teleosteans illustrate this latter process 
also, is probably true. In a section of an early blastoderm (PI. II. fig. 15, a) the infold- 
ing has apparently begun at one point, but the cells of the single stratum — becoming 
crowded together — lie over each other so as to produce a multi-layered appearance (hyp). 
The layer inflected is, however, the outer or corneous layer, as Gotte holds, and this 
point is of some importance, for many authorities who favour the invagination-theory, 
differ as to the layer that undergoes inflection. Thus Henneguy, Agassiz, Whitman, 
and others, though holding strongly to invagination, declare that the outer layer is not 
concerned in the process — a linear fissure, it is maintained, wholly separating the lower 
or sensory epiblast from the outermost layer, the latter indeed ceasing at a certain 
distance from the margin. That the outer or corneous layer alone is inflected is the view 
of Kingsley and Conn (No. 78, p. 201) and others. Teleostean blastoderms are 
particularly unfavourable for deciding critical points such as this, the cells of the various 
layers being almost destitute of those peculiar distinctive features shown in many other 
groups, and an element of uncertainty must necessarily be connected with such a point 
as this. So far as Henneguy's view (No. 64, pp. 402-3) depends upon observations 
on the living ovum, it cannot be relied on, for this point must be determined by sections. 
If Oellacher's well-known figures be referred to, we find in very early blastoderms that 
not only is the epiblast shown extending quite up to the periphery, but the flattened 
cells pass beyond the limits on to the surface of the yolk (No. 114, cf. figs. 4, 5, 6, 
Taf. i.); but such an extension beyond the margin of the blastoderm does not take place 
in the ova dealt with here, though the limits of the germ in section are difficult to 
distinguish, save in such a section as PI. II. fig. 15, a. 

In the living egg a fissure certainly can be distinctly made out, but it apparently 
ceases before the margin is reached. Optical considerations, again, would favour this. 
Henneguy, however, also urges that even in sections this point may be wrongly inter- 
preted, as chromic acid preparations show the same appearance as that we have just re- 
ferred to, and the obliteration of the fissure he attributes to the reagent. The view 
has been suggested (No. 122, p. 449), that while the process is one of invagination, it is 
more than that, since it embraces also a species of budding, such as Lereboullet alludes 
to (No. 94, p. 253), cells segmented from the periblast being added to the blastodermic 
margin, and folded in along with ectodermal cells. This vegetation of periblastic cells 
will probably be most active along the posterior edge of the scutum, but no evidence of 
this is indicated until a later stage. The entire rim is thus a region where peculiarly 
complex processes are going on, for not only is the outer edge continuously progressing 
towards the vegetal pole, but the inner edge is also advancing towards the opposite 
pole, and this is rendered possible by the combined inflection of epiblast-cells, and the 
inclusion of periblast-elements. It appears that Kingsley and Conn, while holding that 
the epiblast is really inflected as stated above, also regard the intermediary layer as 
adding cells to the invaginatecl hypoblast (No. 78, p. 209). The inflected cells creep up 


as a single layer, except at the margin where they are heaped together (PI. II. fig. 10), # 
are very much flattened towards the animal pole, and merge with the cells from other 
parts of the rim. The effect of this union (especially where the cells from the rest 
of the rim meet the cells of the scutum as it proceeds towards the same pole, as well 
as laterally) is, that the original very definite outline of the shield becomes irregular, 
and finally almost wholly disappears. The rim, however, does not vanish with the 
appearance of the carina, as Kupffer and Van Bambeke hold, nor are the two structures 
really so intimately connected as is often supposed. The rim continues even after the 
alar expansion of the scutum, for the reason just stated, is no longer visible. The shield, 
in fact, exists before invagination of the hypoblast, if by the shield be really meant the 
embryonic thickening, and not merely a visible scutiform appearance; but it passes 
insensibly away on all sides, save posteriorly. The invagination-cells do not so much 
produce the shield or carina as make both optically visible.t 

The ectodermal and periblastic cells, which are inflected, result in the establish- 
ment of a single layer of flattened cells — a sheet, in fact, of continuous hypoblast, 
which, as Haeckel held (No. 63, p. 91), limits ventrally the embryonic lamella. It 
separates the carina from the yolk, save in the caudal region, where sections even more 
than the study of the living ovum indicate the special activity which centres there. 
It is noteworthy that the rim does not contain any mesoblastic cells, as in Rana, the 
Teleostean resembling the Cyclostome (Petromyzon) in this feature. In the region of 
the scutum the hypoblast, of course, includes in its fold lower layer cells, but their 
significance at this time is indifferent. This view, we think, explains satisfactorily the 
origin of the primary rim, the thickening of the blastoderm, the extension of both, the 
definition of the embryonic scutum, and its subsequent gradual disappearance. At any 
rate, it is difficult to explain these phenomena by any process of delamination such as 
that of Oellacher, Kyder, and others : differentiation in situ of the lowest stratum of 
the primary entoderm would hardly produce the definitely-bounded thickening, and the 
centripetal progress of the same. The whole appearance and behaviour of the cells of 
the rim in the very transparent blastoderms here considered, strongly suggests invagina- 
tion rather than delamination. Oellacher's figures (No. 114, Taf. i. and Taf. ii.), it is 
true, as strongly indicate delamination, though figs. 2 and 3, Taf. i. might represent an 
inflection of the lowest layer. At a later stage, when Oellacher recognises a definite 
" unteres Keimblatt," the cells are rounder and larger than the superjacent cells, a 
condition quite the reverse of that which obtains in the Gadoids. It would appear as 
if the character of the constituent cells of the hypoblast in these groups were not only 
thus unlike, but that in its mode of origin very marked differences also existed. Mr 

* This centripetal passage of cells, there can be little doubt, is of profound ancestral significance; it can be no less 
than " a real survival of the hypoblast cells to grow inwards during the process of involution " (Balfour, he. cit., p. 530). 

t The curious notions of Oellacher (vide No. 113, pp. 21, 40) respecting the various shapes assumed by the scutum 
at different stages, do not seem to be borne out by study of Gadoid and other forms ; and the opinion formerly expressed 
by one of us (No. 123), that the shield shows differences in outline, characteristic of different species, also needs 


Cunningham's suggestion may indeed precisely express the fact, when he hints that 
this layer may be produced in Salmonoids by delamination, and in the Gadoids and other 
forms by a centripetal process (No. 48). 

In either case the final result is the establishment of a continuous layer of flattened 
cells, which extends underneath the blastoderm, and forms an alar expansion on each 
side of the trunk of the embryo. Agassiz and Whitman speak of it as three or four 
cells deep below the embryonic axis ; but this is true only for a slightly later stage, after 
proliferation has commenced. A typical section of the Teleostean on the establishment 
of the hypoblast, i.e.. when the yolk is about half covered, shows (as in PL II. fig. 17) a 
single-layered corneous epiblast, ep, formed of fusiform or flattened cells, which roofs over 
a thick mass of cells for the most part derived from a second layer of epiblast, the 
sensory or neurodermal stratum, 11, and lastly, the single layer of cells composed of the 
invaginated hypoblast, hy. The more or less acuminate snout of the embryo often appears 
to dip into the hypoblast in front, or rather the hypoblast (hy) seems to creep up and 
overlap the anterior end of the embryonic carina, car. (PI. III. figs. 5 and 6). Posteriorly 
the hypoblast does not exhibit the flattened or squamous character, but forms a small 
tract of full, conical or cubical cells, hy (PI. IV. figs. 56 and 6). These cells, which are 
quite at the blastoporic termination of the embryo, arch over a horizontal cavity, and 
form indeed a superior enteric roof, constituting, as Cunningham strongly and ably urged, 
a plate of dorsal hypoblast, and giving origin, as will be shown, to the notochord. 
These two important points fall to be considered shortly. 

The germinal area after completion of cleavage may be said to present three successive 
phases, — first, it is composed of archiblast cells (PI. II. figs. 1 and 2) of fairly uniform 
size, polygonal, uninucleate as a rule, and formed of clear protoplasm free from yolk- 
spherules ; secondly, an ujyper stratum becomes slightly flattened, ■ and may be dis- 
tinguished as ectoderm, ep (PI. II. fig. 3), while the mass of unaltered cells below forms 
the " lower layer " or primitive entoderm, 11 ; thirdly, the ectoderm, though at first a 
single layer, subsequently exhibits three or four layers, and the outer stratum is the 
epidermal or corneous epiblast (" Hornblatt," Oellacher, " Umhiillungshaut," Reichert, 
" Deckschicht," Gotte); while the under stratum, which always consists of more than one 
layer of rounded cells, is the sensory epiblast, 11 (Sinnesblatt of Oellacher), and this 
latter layer by rapid proliferation forms the neurochordal carina, constituting the main 
mass of the embryonic thickening, which below is limited by the single hypoblastic stratum, 
hyp. These three stages are represented in PI. II. figs. 2 and 3. 

Epiblast. — Little can be added by way of special remark in regard to this layer. 
Certainly the late distinct differentiation of the epiblast in Teleosteans forms a point of 
contrast to the condition in Elasmobranchs and Amphibians ; but Ryder's statement 
that the epiblast, with the other germ-layers, is only split off when the shield appears 
(No. 141, p. 494),* will not apply to the forms mainly treated of here, for the epiblast is 

* Lerkboullet also in his forms (Perca and Esox) made out his epidermoidal layer only when the equator was 
reached (No. 93, p. 493). 


visible, and is inflected as the peripheral rim when barely one-tenth of the vitellus is 
covered, whereas fully a sixth is enveloped before the expansion of the shield is indicated. 
When it first appears the outer layer is distinguishable only by the slightly depressed 
appearance of its cells. It is a single layer, and is difficult to make out, as it does not 
present the regular disposition or columnar character of the ectoderm in other forms. 
The second stratum is well marked when the blastoderm extends over a quadrant, and, 
as already pointed out, its cells are not at all depressed, but are rounded or polygonal, and 
form several layers — indeed, they are distinctly marked off from the corneous layer. The 
existence of this layer has been disputed by Haeckelhi these words — " I do not consider 
the idea of a special nervous layer many embryologists separate from the cuticular 
sensory layer to be confirmed ; " # and Kupffer denies that this layer exists laterally, 
for he distinguishes the corneous stratum only, and indeed doubts the presence of a 
median sensory layer as such, the outer epiblast appearing to him to merge in the neuro- 
chordal mass below, as though it alone gave origin to it (op. cit., p. 243). 

Mesoblast. — The origin of the mesoblast is still a point affording matter for discussion, 
but the Teleostean blastoderm, it may be readily surmised, does not offer great facility 
for deciding the matter.t That it is not a primitive layer, but is derived from one of 
the primary layers, i.e., ectoderm or endoderm, is beyond dispute. 

Lankester seems to have been the first to suggest that, viewed phylogenetically, the 
mesoblast arose as a paired outgrowth of the entoderm, a fact which Kowalewsky had 
ascertained to be true for Sagitta (No. 85, p. 827). 

In the Mollusca and Annelida we know that the mesoblast usually arises not as a 
single sheet, but as two distinct masses, just as in Amphioxus and many Craniates. 
Thus Scott and Osborn found in Triton that the two bilateral masses were invaginated 
as such, and were never confluent in the middle liue, the axial epiblast and hypoblast being 
only in contact along that line (No. 147, p. 455). Scott also affirms in Petromyzon that 
some mesoblast (dorsal) is invaginated with the cells of the mesenteron, while the cells of 
the ventral mesoblast are derived from the superficial cells of the yolk; but Shipley's later 
investigations have demonstrated that in this form no mesoblast is invaginated, the two 
longitudinal bands being differentiated in situ (No. 149, p. 244). Balfour showed, and he 
is confirmed by His, that in Elasmobranchs the two bands arise in the manner just stated 
(No. 14, pp. 35-56); but in Lepidosteus Balfour and Parker give no account of the origin 
of the mesoblast. In certain Teleosteans, Haeckel describes a bilateral development 
(Jenaische Zeit., Bd. ix.), while Kowalewsky says it originates from an invagination 
of the embryonic rim (No. 86). In speaking of the epiblast, it was indicated that our 
observations do not show such an inclusion of mesoblast by the reflected layer of the 
blastoporic lip ; and unlike the condition in Rana and other forms, the infolded layer, hyp 
(PI. II. fig. 15<x), is in close apposition to the epiblast, ep, above. In the middle line 

* " Gastraea Theorie," see Quart. Jour. Micr. Sci., vol. xiv., note on p. 32. 

t It need hardly be pointed out that in so familiar an ovum as that of Rana, the precise origin of the mesoblast is 
really undecided, and it is still to be settled whether the layer is derived from the " intermediary " mass of small cells, or 
from the endoderm by proliferation, as seems more probable. 


of the embryonic thickening, the proliferated epiblast, ne (PL III. fig. 1), and the lower 
layer cells, of course, lie above the invaginated hypoblast, hyp. These lower layer cells 
probably become largely converted into mesoblast, though it is certain that the hypoblast 
also buds ofF some mesoblastic cells. W. Wolff has recently expressed a view similar to 
this, though he denies that the mesoblast (Mittelkeim) arises in any way from the endo- 
derm. The cells which build up the mesoblast represent, he holds, the surplus of those 
blastomeres which are not used in forming the gastrula (No. 160, pp. 425-448). Accord- 
ing to Kupffer, His, and Klein, the mesoblast results solely from the differentiation of 
the deeper germ-layer, while the hypoblast is stated to originate in the periblast (Klein's 
" parablast "). Gotte speaks of it as formed from the invaginated layer, which gives 
origin in addition to the hypoblast. The fact would seem to be that much mesoblast is 
formed from the lower layer cells, // (PI. II. fig. 15), these cells being a continuous 
sheet, viz., the primary entoderm of the early two-layered blastoderm, and they become 
severed into two longitudinal masses, mes (PI. III. figs. 2 and 11; also PL IV. figs. 5 and 
10), by the proliferation of epiblast, ep, which produces the medullary plate, or 
neurochord, ne. The sub-ectodermic mass, 11 (PI. II. fig. 15), cannot be regarded as 
mesoblast until it is severed mesially — the mesoblast, when recognisable as such, is defined 
as two lateral plates, just as in Petromyzon (Calberla), Triton (Scott and Osborn), 
Elasmobranchs, and other forms. Kingsley and Conn speak of this continuous sheet, at 
an early stage, but their figures are not decisive. Thus their fig. 25, to which they 
specially refer, as also figs. 26 and 27, show a massive dorsal plate, which must be the 
thickened epiblast, i.e., the neurochordal proliferation, and against it the notochord abuts 
below. The mesoblast must, in part, constitute the lateral plates, though the authors 
themselves do not so interpret their figures. This interpretation appears, in fact, 
irresistible, though it is not in agreement with the view stated in the text (No. 78, 
p. 200). Ryder* records a peculiar condition in Elecate, viz., a precocious metameric 
segmentation in the two parts of the rim which diverge from the posterior end of the 
trunk. This is very remarkable, for no such feature has been seen in any other form, 
while in those referred to in this paper, the posterior portion of the trunk, after the 
mesoblastic plates are defined anteriorly, shows no such differentiation, the three layers of 
the mid-region merging, in fact, in a mass of indifferent cells at the posterior termination 
(vide — prs, PI. III. fig. 12, and PI. IV. figs. 5d and 5e). These two mesoblastic plates, as 
seen in section mes (PL III. fig. 11), have above a thin covering of epiblast, ep, and interiorly 
an insinuating layer of hypoblast, hyp, which separates the embryo from the yolk below. 
Anteriorly the mesoblast thins away, and in the otocystic region is reduced to a single 
layer of somewhat depressed cells, mes, between the hypoblast, hy, and the greatly 
enlarged neurochord, mo (PL IV. fig. 4). In Oellacher's figures of the trout at this 
stage, the mesoblast is not so much reduced ; but its larger bulk is probably connected 

* Ryder's view of the origin of the mesoblast is not clear ; he apparently favours delamination with Oellacher 
(op. cit., pp. 494-95), and hypoblastic proliferation (on p. 570); while on p. 501 he seems to suggest a sundering of the 
"lower layer" mass, such as is insisted upon above. 


with the diminished neurochordal mass (vide No. 114, Taf. iii. figs. viii 4 and ix x ) in that 

Further forward (PI. IV. fig. 3) it apparently ceases altogether, the cells beneath the 
optic vesicles, op, being hypoblastic, while the denser stratum, ep, above, is neurodermal 
(sensory epiblast), unless the small strand of cells filling up the triangular fissure on each 
side be a continuation of the mesoblast behind (marked in the fig. mes ?). Oellacher's 
representation of this region is not unlike our fig. 3, PL IV., but here again mesoblastic 
cells are shown as somewhat abundant ; his mesoblastic " Kopfplatten " consisting of 
three or four layers, which continue laterally as flattened peritoneal plates. This latter 
structure is wholly absent in our forms, the marginal alse being simply epiblast and 
hypoblast, though the very minute group of cells mentioned (mes ? PL IV. fig. 3) may 
represent Oellacher's cephalic mesoblast. Our figures (PL IV. figs. 3, 4, 16, and 16a) 
support the view that the mesoblast does not yet extend into the head-region, the 
cells at x and y being obviously neurodermal. If the foregoing conclusion be correct, 
the mesoblast arises for the most part in situ from the lower-layer cells in the trunk- 
region proper — that is, excluding the pre-otocystic and caudal portions — by a process not 
of delamination purely, but of mechanical separation, the intruding neurochordal cells 
from above actually pushing aside the subjacent cells as two longitudinal lateral plates. 

It is not easy to see why mesoblastic cells should, as appears to be the case, be absent 
so largely from the cephalic region. Their absence would be accounted for if the 
mesoblast be really a forward growth from the trunk-region, and most probably also from 
the posterior mass of indifferent cells. Such a forward growth has been regarded as the 
sole process of mesoblastic growth (Kolliker, No. 81) ; and if in its differentiation the 
mesoblastic cells are separated at first just in front of the primitive streak, it will be diffi- 
cult to show that some such process of forward growth is not involved. The cells, in fact, 
below the primary ectoderm form a median layer, when the rim is first invaginated below 
it, and since Balfour and Deighton find in the chick (No. 19, p. 180) that the main 
mass of the posterior indifferent cells (primitive streak) is really produced by epiblastic pro- 
liferation, it follows that some mesoblast is really indirectly of epiblastic origin. Bambeke, 
indeed, regards the mesoblast in Teleosteans as the lowest delaminated stratum of the 
primary upper layer of the germ (Von Baer's animal layer), i.e., ectoderm. This upper 
layer in his view divides into three, viz., the corneous, neurodermal, and mesodermal 
layers (No. 20a, pi. iii. fig. 8, pp. 57-58). Delamination solely will not account for the 
fact that in Teleosteans the mesoblast is certainly best developed in the posterior region,* 
as would be implied by the theory of forward growth, and we see that it thins away 
anteriorly. A comparison of figs. 3, 4, and 5a-5c, PL IV., sufficiently demonstrates this. 
Even at a later stage the same feature appears (see figs. 10 and 11, PL IV.), as though 
the mesoblast in extending anteriorly into the head receives continual additions from 
behind. In Petromyzon, Shipley, indeed, regards the muscular elements of the mouth 

* This is also the condition in Elasmobranchs, the mesoblast being accumulated at the posterior end as prominent 
tail-buds (loc. cit., p. 557). 

VOL. XXXV. PART III. (NO. 19). 6 A 


and gills, besides the eyes and mouth themselves, as developed from wandering mesoblastic 
cells as well as unsegmented mesoblast (No. 150, p. 336), and these wandering cells 
"Wenckebach has recently affirmed to be active in Teleosteans in building up the heart 
and its connected trunks, and other parts of the embryo (No. 158). It cannot be 
denied that in fig. 2, PI. III. and figs. 4, 5a, and 56, PI. IV., the mesoblast has more 
intimate relation to the hypoblast than to the epiblast, and the condition presented by 
these early sections corroborates the view that the mesoblast is of hypoblastic origin, as 
Gotte strongly holds (No. 58). That the mesoblast in the Teleostei has in fact a three- 
fold origin is consonant with the figures given in various plates, — part being formed 
directly by conversion of lower layer cells in situ, while part is proliferated from the 
invaginated hypoblast beneath, and lastly to make up for the forward growth of these 
cells into the cephalic region, other mesoblastic cells are derived from the indifferent mass 
constituting the caudal region. It is singular that this account of the multiplex growth 
of the mesoblast should coincide, even down to many details, with the derivation of this 
layer in the chick, according to Balfour and Deighton. In their paper (No. 19) part of 
the mesoblast is determined to be from the indifferent cells of the primitive streak, prim- 
arily epiblastic {Ibid., p. 182); some mesoblastic cells, which are stellate, are differentiated 
from the hypoblast (pp. 184-5); while certain others lying below the epiblast in the early 
blastoderm (see No. 11, fig. 91, I, p. 150), and really "lower layer" cells, Balfour con- 
siders "have also a share in forming the future mesoblast" (p. 154). Kingsley and 
Conn, though they furnish no account of the process, come to a similar conclusion, and 
hold that this middle layer is derived partly from hypoblast and partly from lower layer 
cells (No. 78, p. 200). 

Hypoblast. — The hypoblast, hy, which there can be little doubt is pushed in 
from the periphery as an inflected layer of ectodermal, for the most part " corneous 
layer" cells, ep, with some cells derived from the periblast, per, insinuates itself 
between the under surface of the germ, 11, and the cortex of the yolk, y, forming the 
limiting layer on the ventral aspect of the embryo. It separates the neurochord {ne, 
PL IV. fig. 5a) in the middle fine and the lateral cells, mes, destined to form, in part, 
the mesoblast, from the yolk, y. It remains for some time as a single layer of flattened 
cells, hy, in the anterior and mid portions of the embryo ; but at the posterior termina- 
tion (PI. IV. figs. 5d and 6) its character alters, for it is there less definite, merging, in 
fact, with the heaped-up periblast, per, like the thickened layer of dubious cells, which in 
the chick continue into the "germinal wall" behind (No. 19, p. 179). This tract of 
mingled hypoblast and periblast is the site of much developmental activity, and about 
the time that the blastopore closes it becomes defined as a bridge of swollen columnar 
cells, hy, in the median line, arching over a fissure below, and pressing against the 
neurochord, ne, above (figs. 56 and 6, PI. IV.). We see here the very phenomenon which 
Kingsley and Conn* and Cunningham have suggested, viz., that the invaginated hypo- 
blast is really " dorsal hypoblast, roofing over a primitive enteric cavity, whose floor is 

* Op. rib, p. 201. 


granular protoplasm with many nuclei, and cells apparently forming around them." 
Anteriorly, the hypoblast still preserves its flattened character (hy, PL IV. fig. 5a), 
while in the otocystic region it seems to merge in the neurochordal cells, ne, unless the 
undefined cells in the middle line be a thin stratum of mesoblast, in course of formation, 
and destined partially to constitute the nuchal and cephalic mesoblast (PL IV. fig. 4). 
A similar indefinite axial tract occurs in the chick (No. 19, p. 184). Further forward the 
hypoblast is once more fairty defined (PL IV. fig. 2), and at the tip of the snout, as before 
mentioned, it may often be distinctly seen to overlap the epiblast as a thin veil (figs. 4-6 
and 19, PL III.). 

The Blastopore. — The blastopore (Dotterloch — trou vitellaire) may be said to exist 
from the moment that epiboly begins. It coincides with the margin of the germ, and 
forms in fact the border of the saucer-like blastoderm at the conclusion of cleavage. 
Later, however, it is more distinctly recognisable as a kind of spacious mouth, from which 
the ball of yolk is seen projecting. Small granules often occur plentifully at the margin 
of the rim, and are imbedded in the periblastic ring (PL III. fig. 16). The continued 
extension of the germ over the yolk produces certain changes, notably in its diameter, 
which are easily observed. 

Contrary to Oellacher's view, the rim seems to progress at an equal pace at all 
points, and it thus increases in diameter until the process of enclosure is half accom- 
plished; but after the equator is passed, the aperture necessarily diminishes, and finally 
presents a fairly circular form. Oellacher regards the caudal end of the embryo as 
a fixed point, so that the parts of the rim further away from this point advance at an 
increased rate — progress being, in fact, rapid in proportion to their remoteness (No. 114, 
p. 4). This assumption, however, is very questionable, the snout of the embryo being 
apparently the fixed point, while increase in length takes place in the caudal region. 
No part of the rim can be shown to be stationary, for the embryo lengthens as epiboly 
proceeds, and no part presents more signs of active growth and development than the 
posterior extremity, as already indicated. 

The lip of the invaginated rim, for which the name blastopore is on every ground 
justifiable, attains its maximum size when the equator is reached, and after that stage it 
continues to diminish until finally it closes. Often it assumes an oval form (bp, PL III. 
figs. 7, 23 ; and PL XXVIII. fig. 5), doubtless due to the plastic nature of the yolk; but 
usually an almost perfectly circular outline is preserved. In some cases the blastopore 
has the rude outline of a flask, the narrow portion forming a bay, which coincides with 
the caudal end of the embryo, and this has suggested the theory of concrescence in these 
forms. In most cases no such terminal bay is seen, the embryo in fact projecting more 
or less prominently, and breaking the circular outline of the blastopore in a manner 
exactly the reverse of that just mentioned. In the later stages of development in 
ovo the concrescence theory is not clearly borne out, e.g., by the view of the gurnard 
(PL XIV. fig. 7), in which the rim forms a backward loop at the tail. This concrescence, 
however, may occur without a visible bay or angle directed forward, as indicated by 


J. T. Cunningham. Again, it is observed that towards the closure of the blastopore the 
" limbs " of the blastoderm seem to go — so far — into the embryo. When this projection is 
less marked the caudal end of the embryo may still destroy the regularity of the 
circumference, as in PI. III. fig. 23, recalling the horse-shoe-shaped blastopore of Astacus, 
such variations being easily explained by the bulk of the contained deutoplasmic matrix 
and the tension of the blastodermic membrane. This pressure outwards, as Van Bambeke 
pointed out, and the restraint of the blastoderm, frequently produce a contracted opening, 
like the mouth of a balloon (see Van Bambeke's figure, No. 20a, pi. ii. fig. 9), from 
which a plug of yolk protrudes, just as in the Crustacean ovum, mentioned above, an 
endodermal protrusion fills up the blastopore. In Teleosteans, as in Astacus, the plug 
diminishes as the blastopore closes. In the gurnard, as the blastopore closes, projecting 
cells are seen, which often send out protoplasmic processes, those protruding from the 
blastoporic lip somewhat resembling the processes which under pressure are pushed out 
from the marginal cells of the blastodermic ring at an earlier stage (PI. II. fig. 16). The 
time of the closure of the blastopore of course varies, according to circumstances, in 
common with the other features of development. Thus in Trigla gurnardus the closure 
was observed to be effected on the third day after fertilisation ; whereas in another series 
earlier in the same season (May), the temperature being lower, this did not occur until 
the fifth day. As closure takes place the yolk may often, in side views, be seen still to 
project as a diminished yolk-plug (PI. III. fig. 15) ; but usually as closure is effected 
the blastopore forms a trumpet-shaped opening, round which the deeply corrugated lip 
rises as a circular eminence (PL III. figs. 9, 10, 21). 

Kupffer's Vesicle. — When the blastopore closes, or often a few hours earlier,* a minute 
vesicle arises on the ventral aspect of the embryo slightly anterior to the caudal termina- 
tion. Its advent is preceded by the appearance, in some cases, of vesicles or small 
elongated spaces (PI. III. fig. 17), evidently filled with colourless or pinkish fluid. They 
occur quite at the margin, as if the advancing embryonic area became elevated at these 
points, and progressed over them. In other cases a granular thickening occurs in which 
a few rounded vesicles are imbedded, as can be readily seen in Trigla gurnardus and 
other forms shortly before the blastopore closes. Kingsley and Conn noted such a group 
of minute vesicles, which in five hours apparently by coalescence showed the characteristic 
form and appearance of Kupffer's vesicle. It is defined in their figure, above by hypo- 
blast, and below by periblast (No. 78, pi. xvi. fig. 54). It is variable in the precise time 
of its appearance, for Henneguy noticed it in Salmo fario when only about half of the 
vitellus was covered by the blastoderm (No. 80). In Molva vulgaris, Gadus morrhua, 
and other species it is usually not visible during the open state of the blastopore, but 
both in position and time of its appearance it varies, though the clear vesicular structures, 
with a delicate envelope, are usually exhibited. Kupffer, who first described it in 
Gastrosteus, Gobius, and others, calls it the " allantois," and says that it acquires a coating 
of cylindrical epithelium, and finally becomes the bladder, though he did not show how the 
* J. T. Cunningham found that in Clupea it was late in appearing (January 1886). 


primary " Urnierengange " communicated with it (No. 88). Henneguy also speaks of a 
cellular wall, but it appears to be more truly a wall of clear protoplasm in which nuclei 
rapidly develop, and not wholly a wall of cylindrical cells. In regard to form, it may be 
more or less spherical (kv, PL XXIII. figs. 8, 9), or markedly ellipsoidal (PI. XXII. fig. 12), 
this latter figure being frequently altered by the flattening of its floor (kv, PI. III. figs. 
21, 22) and the increased curvature of the roof, — changes best seen in side views; while 
again its shape may be wholly irregular (PI. III. fig. 14); or lastly, it may simply take the 
form of a sub-embryonic fissure. Secondary vesicles are very frequent, and they present 
the same features as the normal vesicle (PI. XXIII. fig. 9) ; but may extend all along the 
ventral line almost to the pectoral region. In the gurnard this multiplicity of vesicles 
is often a very striking feature, whether extending along the sub-alimentary region, or 
accumulated together as a prominent cluster of bubble-like structures. A small anterior 
vesicle in addition to the normal one is often seen (PI. III. fig. 20, and PI. XXIII. fig. 8), 
and a connecting granular strand, but there is no apparent tendency to amalgamate. 
The diameter of the larger vesicle in an example of Gadus ceglefinus was found to be 
'005 inch, but occasionally, as in Trigla gurnardus (third day), the vesicles which form 
a group may even be five or six times larger than the ordinary vesicle. An embryo 
of G. ceglefinus was observed to exhibit one or two small vesicles near the large 
vesicle, and three hours later, the large or normal vesicle and one of the smaller were 
almost free from the embryo, being in fact pressed into the surface of the yolk. 
Other three vesicles had developed and occupied the region whence the large vesicle was 
protruded, and shortly after, on viewing from above, the vesicles were seen to be upon 
one side of the trunk, viz., that to which the tail was bent. Still more remarkable was 
the situation in some examples of G. morrhua, for just before the blastopore closed, 
in addition to the ordinary vesicle, a large clear vesicle also occurred midway along 
the trunk, and it deeply indented the yolk. Moreover, a vesicle also appeared at the tip 
of certain protoplasmic pseudopodia which were pushed out from beneath the embryonic 
trunk. In another example, Kupffer's vesicle was situated posterior to the caudal termina- 
tion upon a process of protoplasm. Agassiz and Whitman called attention to appearances 
similar to the foregoing (No. 2, p. 73), designating them " secondary caudal vesicles," 
and observing that they differed little if at all from Kupffer's vesicle. Whatever signifi- 
cance be attributed to this latter structure, it is in any case simply a fissure or cavity 
beneath the embryo (see section kv, PI. IV. fig. 56), and is defined usually by the dorsal 
hypoblast, hy, above, and the periblastic matrix, per, below. Its contents are usually 
homogeneous and clear, evidently a translucent plasma, though occasionally granules 
find their way from the basal portion of the vesicle into its lumen. Such being its 
structure, it is not remarkable that it should vary in shape, or often be a compound 
instead of a single vesicle. Balfour (No. 11, p. 61), Rauber (No. 133), and Balbiani 
(No. 9) favour the view that it is of ancestral value, and represents the invaginated 
enteric cavity of Cyclostomes and Amphibians.* Henneguy could not make out any 

* See also a discussion on the subject by J. T. Cunningham (Quart. Jour. Micr. Sci., January 1885). 


canal connecting it with the exterior, either in transverse or longitudinal sections; 
but sections cannot satisfactorily demonstrate this point, the vesicle itself being evanes- 
cent, and its walls of delicate protoplasm are so readily affected by reagents, that a 
minute fissure is easily reduced or closed, so as to be indistinguishable. Study of 
the living condition is therefore most reliable upon this point, and it must be observed 
that Henneguy did make out a canal connecting the vitellus with the dorsal surface of 
the embryo ; but he regards it as wholly independent of Kupffer's vesicle, for this latter 
structure, he says, has disappeared some time before. But in so delicate and transitory 
a structure as this vesicle, it is important only that its site should be regarded, and there 
can be no question that such a posterior canal passing to the yolk beneath the embryo is 
in communication with that site, even though the vesicle itself be no longer distinguish- 
able. The enteric cavity at this stage is little more than a fissure between the (dorsal) 
hypoblast and the yolk-cortex or periblast ; and Henneguy's canal can be no other than 
the post-anal passage trending round from the dorsal groove to the under surface of the 
embryo (that is, the surface of the yolk in Henneguy's view), and connecting the 
transitory medullary groove, with the no less transitory primitive enteron known as 
Kupffer's vesicle. Ryder admits that a neurenteric canal is represented, but not by a 
tubular connection ; the solid caudal mass, where hind gut and neurula mingle, must, he 
holds, in its axial part, represent the canal. But Ryder also noticed a fine canal passing 
from the vesicle to the blastopore, and says — " I reserve my decision as to its true 
nature" (No. 141, p. 527). 

Neurenteric Canal. — As the blastopore closes, a favourable side view of the caudal 
region shows a faintly marked fissure (nee, PL III. figs. 9, 20, and 22), or rather what 
seems to be a tubular connection of the external blastopore and the ventral surface of 
the embryo. Unless the chamber x (PI. IV. fig. 5d), be an artificial product, the 
tubular character is demonstrated in the section. This slight cavity curves downward 
from the blastopore, and widens out laterally beneath the embryo (PL III. figs. 8 and 86), 
passing for a short distance forward as a mere line marked by fine granules, and dis- 
appearing, as Kupffer's vesicle, or the site of it, is reached. Any actual union of the 
two vacuolated spaces is not easily made out, but the merging of the tract just described 
and the protoplasmic wall of Kupffer's vesicle is unquestionable (PL III. figs. 20 and 22). 
In fig. 9, PL III., the course of the canal, nee, from the corrugated blastopore, bp, forward, 
is well seen, but Kupffer's vesicle is not yet defined ; and the relation of the two is better 
seen in figs. 20 and 21, above mentioned, where the vesicle, kv, a minute lozenge-shaped 
chamber, is undoubtedly related to the tract, nee, posterior to it. Certainly the passage, nee, 
in fig. 22, is most readily, and without doubt correctly, interpreted as a neurenteric canal. 

The existence of such a canal in Teleosteans has often been questioned, and, indeed, 
Miss Johnson amongst others declares that no such structure is known in these fishes, 
nor an invagination giving rise to a blastopore (No. 76, p. 666); though Kowalewsky 
is stated to have announced in an early volume of the Arch. f. Mikr. Anat. (vol. vii. 
p. 114) such a connection of the alimentary tract with the dorsal groove in Teleostei; and 


Kingsley and Conn refer very briefly to what they style a " neurenteric canal," of 
which they give a figure (No. 78, fig. 30, pi. xv.). Raffaele also recently alluded to it 
in Uranoscopus (op. cit., p. 28). That it has been rarely observed, and never fully 
described, is probably due to its evanescent character, and it may in some cases, indeed, 
never be developed. Balfour and Deighton (No. 19, p. 185) speak of it as "that 
most variable structure in the chick," and the same description may be applied to 
it in the Teleostean ovum. This canal can hardly be due to the supposed process of 
concrescence, as it has not the character so much of a vertical fissure as a depressed cavity 
passing obliquely downward and forward between the embryo and the yolk, and is best 
seen in transverse or side view. It is, indeed, less of a tubular canal than of a tranverse 
fissure between the convex embryonic surface and the concave yolk-surface, and opening 
externally by the blastopore. In PI. III. fig. 8, in the living condition its course is 
clearly indicated, the shallow dorsal groove continuous with the blastopore indenting 
the caudal region, and then merging in the descending tract, nee, which widens out and 
becomes lost in the mass of periblastic protoplasm, kv, in which Kupffer's vesicle 
makes its appearance. Sometimes this neurenteric passage connecting the neuro- 
chordal groove above and the enteric region below is a distinct interspace (PI. III. fig. 
9, and possibly nee? PI. IV. fig. 5d). It is often marked by granules (PL III. fig. 
22), or even a tract of undifferentiated protoplasm, in which two or three clear spheres 
are imbedded (PL III. fig. 20). Fig. 8, PL III., for instance, showed this last named 
condition at 10 a.m., with a connecting tract opening externally between the closing lips 
of the blastopore. An hour and a half later, a spindle-shaped plug (PL III. fig. 8a) 
sending outward an acuminate process, interrupted the canal, nee, and presented 
amoeboid movements. The plug then coalesced with the margin of the blastopore, and, 
assuming a distinctly granular appearance, formed a bridge across the fissure connected 
with the inferior tract (fig. 8c). # Meanwhile, the clear vesicles mentioned above had 
enlarged, and finally coalesced to form Kupffer's well-known structure. Such a plug 
as we have described Balfour and Deighton noted in the chick, and they speak of a 
mass of rounded cells pushed up through the neurenteric canal (No. 19, p. 186). The 
phenomenon just detailed shows two important points, viz., the connection of the 
external blastoporic orifice with the region of Kupffer's vesicle, if not with the actual 
structure itself, and the obliteration of the passage of connection, i.e., the neurenteric 
canal, by a plug probably pushed up from below. 

The section figured in PL IV. fig. 5d, and already referred to, passes through the 
precise region we have been dwelling upon, and a few loose cells alone obstruct the 
connection of the dorsal and ventral (enteric) groove, ne. The section is interesting as 
showing a portion of Kupffer's vesicle, or the groove itself imbedded in a thick layer of 
periblast, per, as we have before described. 

Now the sections figured (PL IV. figs. 5b-5d, and fig. 6) clearly show the continuity 

* Fig. 86 is an intervening stage, when neither plug nor connecting bridge are visible. 


of the enteron formed by an arch of columnar hypoblast, hy, and a floor of nucleated 
(periblastic) protoplasm, 'per, the ill-defined ascending interspace or canal, nee, being 
bordered by indifferent cells, and opening by means of the blastopore into the dorsal 
groove above. This dorsal groove is more fully treated of on another page, and it can 
be no other structure than the primitive involution forming the medullary canal in so 
many forms, but in Teleosteans simply appearing as a transient, ancestral reminiscence, 
and, except for this, now obliterated. Certainly its connection with the subsequent 
permanent neural cavity cannot be demonstrated. 

So rapidly does the dorsal groove become effaced that in a large series of sections of 
early stages none indicate this structure favourably; but a reference to Oellacher's well- 
known figures (No. 114) sufficiently shows this, the deep groove in fig. iv. 3, Taf. ii., 
being merely indicated in fig. vii. 5, Taf. iii.; while the figs, in Taf. iv., such as fig. iv. 1, 
show no trace of it, nor can the permanent cavity be said to be more than foreshadowed. 
Owing to the rapid and complete obliteration of the medullary groove, the absence of a 
post-anal canal has been generally accepted for Teleosteans, and for this reason Balfour, 
though adding a query to his cautious statement, concluded that no neurenteric passage 
was " apparently developed" (No. 10, p. 286). Balfour and Parker (Phil. Trans., 1885, 
ii. p. 365) speak of the neural canal arising in Lepidosteus as a slit-like lumen, and not due, 
as supposed by Oellacher for Teleostei, to an actual absorption of cells. " When first 
formed, it is a very imperfectly defined cavity, and a few cells may be seen passing right 
across from one side of it to the other " (fifth day after impregnation). The connection in 
Teleosteans between the primitive enteron, no other than the gastrula-cavity (see page 
713), and the primitive dorsal groove cannot be questioned if our interpretation of figs. 
9, 21, and 22, PI. III., be correct, for the continuity of this groove, nee, and the blastopore, 
bp, is very apparent. The formation of a neural canal by a dehiscence of neurochordal 
cells is a secondary process, and the Teleostei therefore form no exception to the condition 
which so widely obtains in other Vertebrata, and which was demonstrated by Gasser in 
birds, by Kowalewsky, Balfour, His, and others in Elasmobranehs, by Owsjannikow in 
Oyclostomes, and by Gotte and others in Amphibians. 

Medullary Groove. — The permanent neural canal is formed comparatively late in 
osseous fishes, whereas in most vertebrates its appearance as a groove on the dorsum is a 
very early feature in development. For a short period, soon after the optic vesicles are 
defined, a transient longitudinal indentation passes along the median dorsal line from 
the head to the tail, just as Lereboullet figures (No. 95, pi. ii. fig. 36). It may be 
regarded as actually reaching to the lip of the blastopore, though the depression is 
so slight, in the extreme posterior region, that it is in some cases indistinguishable. 
In Rana at a certain stage the hind part of the neural groove cannot be made out. 
Spencer, however (No. 151, p. 97), found that it extends quite to the caudal margin, 
but in this latter region it is obliterated — the cavity closes up, and the nervous cord 
becomes solid. The hind end of the trunk in the embryonic Teleostean often appears 
like a flattened plate, in which the neurochord spreads out like a spatula (PI. III. fig. 16). 


This flattened condition frequently continues for some time after the closure of the 
blastopore (PL III. figs. 18 and 20). It is merely a shallow groove, barely perceptible 
posteriorly, and does not therefore enclose the blastopore, which remains open for a 
short time, as a pore with a corrugated margin, but in the cephalic region the groove 
forms, as in the gurnard (dg, PI. III. fig. 4), quite a deep fissure, showing itself 
earliest anteriorly, and extending, as Van Bambeke describes, in the form of " a 
slight depression," the " sillon primitif" (see his fig. 12, pi. ii.), to the tail. In the 
forms here considered, the two lateral folds are by no means sharply ridged, and 
viewed from above the furrow is difficult to make out ; and is thus unlike the condi- 
tion in Esox, which Lereboullet says is distinctly marked by two parallel lines 
— the groove being deepest in the mid-trunk, and gradually disappearing before and 
behind (No. 95, p. 516). In the mid-trunk, he remarks, it likewise remains open 
for the longest time (p. 528). This groove is, however, as before suggested, merely 
a reminiscence of the ancestral condition, and wholly disappears chiefly by the horizontal 
widening out of the embryonic trunk as the blastoderm proceeds to envelop a larger extent 
of the vitelline globe.* This is evidently the case posteriorly, but in the head-region 
obliteration is achieved less by elevation of the base of the groove than by coalescence of 
its walls. 

Kupffer maintains that it is not by any means the homologue of the medullary 
groove of higher Vertebrates (No. 87, p. 251); while Oellacher regards it as pro- 
duced by the formation of the carina, the furrow deepening as the keel presses down- 
ward, and it is certainly true that the furrow is produced subsequent to the growth 
of the carina, and does not, as he proved, become the medullary canal ; but the view 
adopted in these pages, that the carina is a neurodermal proliferation and the dorsal 
furrow an ancestral reminiscence, agrees best with appearances in life and in sections. 
Certainly no confirmation is given to Calberla's opinion that ectodermal cells are 
involuted along the central dorsal line to form the epithelial lining of the neural canal, 
as the same authority, supported by W. B. Scott (No. 145), holds to be true for 

As a matter of fact, the dorsal groove in Teleosteans does not appear to become any 
organ, but wholly passes away. It is subject to great variation, just as in the chick, for 
at times it is apparently entirely wanting, or at most is represented merely by a 
shallow depression, which may be discernible in the short posterior part of the indifferent 
caudal mass.J This posterior mass of indifferent cells, to which reference has frequently 
been made, forms the termination of the embryo (PL III. figs. 18, 20-22), where it 
reaches the lip of the blastopore, bp. In it neither neurochord, notochord, nor mesoblastic 

* The superficial extent of the Teleostean emhryo is a characteristic feature, and the dorsal groove is thus opened 
out on account of the large bulk of the yolk upon which the germ lies flattened. Ryder makes a passing reference to 
this (No. 141, p. 564). 

t This epidermic involution in Petromyzon has now been disproved by the recent investigations of Shipley 
(No. 150, p. 9). 

% Compare the observations of Balfour and Deighton on the chick (No. 19, p. 183). 

VOL. XXXV. PART III. (NO. 19). 6 B 


plates can be distinguished, for all these melt into a common aggregation of cells, below 
which even the hypoblast, as Balfour and Deighton note also in the chick (No. 19, 
p. 180), is hardly separable as a denned layer (PI. III. figs. 3 and 12, and PL IV. figs. 5b 
and 5c). The epiblast (ep, PI. IV. fig. bd) laterally is partially differentiated ; but in 
the middle line it merges in the cells below, to which, indeed, it gives origin. All these 
features point to its identity with the primitive streak of higher forms. The primitive 
streak, it is true, according to the accepted interpretation, arose in the process by which 
the embryonic trunk, notably in the Sauropsida, was removed from a marginal to a more 
central position on the surface of the yolk. This transference drew after the embryo, as 
it were, the diverging arms of the blastoporic lip, and their cells form a post-embryonal 
mass, which is the primitive streak. In Rana temporaria, as Spencer found (No. 151, 
p. 97), the point where the medullary groove opens into the blastopore becomes solid, the 
neurochord losing its canal, and the epiblast, mesoblast, and hypoblast fusing as an indif- 
ferent mass just anterior to the blastopore. The Teleostean embryo reaches to the 
periphery of the blastodermic area, and any similar aggregation of indifferent cells is 
reduced to its smallest limit, yet such an aggregation exists, as a transient posterior 
mass, into which the notochord and other structures, anteriorly placed, pass and disappear. 
It is so in the chick, and in both the structure is transient — its importance goes with 
the earliest embryonic stages, and it disappears, or rather is used up, partially as we have 
seen, in the production of mesoblast, and still more by the extension posteriorly of the 
embryonic trunk, and the development of the tail. Its position on the anterior margin 
of the blastopore is easily explained, the present anterior margin is really the primitive 
posterior margin. If the blastopore extended to the ventral surface of the embryo, an 
increase in the amount of food-yolk would cause its true anterior margin to be pushed 
away from the ventral surface, and as it was thus carried outwards, the true posterior 
margin remaining unmoved, the parts of the blastopore would become reversed, just as a 
pendulum, if held horizontally in a north and south direction with the weight north, would 
with the first swing become reversed, the fixed attachment would point north, and the 
weight (i.e., the true north end) would become south, and thus it is that the present 
posterior edge of the blastopore is really the former anterior margin. According to 
this view, we see that the blastopore, having drifted outwards, no longer coincides with a 
ventrally placed anus ; and the relations of the primitive mesenteron, post-anal gut 
(Kupffer's vesicle), and neurenteric connection with the dorsal groove are placed in a 
clear light by means of the blastopore. 


VIII. General Development of the Trunk. 

After the closure of the blastopore, the definition of the embryo as a cylindrical 
rod, pressing somewhat deeply into the surface of the yolk (PI. III. figs. 1-4), 
becomes marked. Anteriorly its enlarged cephalic region soon rises boldly above 
the surface of the periblast ; while the trunk, though prominently standing out 
along the dorsum, and indenting the yolk in a pronounced manner ventrally, yet 
laterally, by the alar expansion of the scutum on each side, gradually merges in the 
general expanse of the blastodermic envelope, as observed in the serial sections 
(figs. 3, 4, 5a-5d, PL IV.). The true limits of the embryonic trunk are in reality 
not defined, the neurochord, ne, and myotomic masses, mes, are distinctly marked, but 
more distally, in the snout and tail, as well as the lateral regions, no sharp line of 
demarcation divides the young fish from the blastodermic area beyond. The embryonic 
Selachian or bird is pinched off more or less sharply at an early stage ; but the Teleostean 
embryo, instead of becoming folded off as it were from the j^olk, continues to lie 
extended upon its surface, and gradually draws the vitellus into its large subenteric 
enclosure, the abdominal walls, as we shall see ultimately, entirely encompassing the 
yolk. In some forms the yolk persists less prominently than in others, the somato- 
pleure more rapidly extending ventrally and enveloping it. It never projects, as in 
Elasmobranchs and Sauropsida, in the form of a dilated sac distinctly separated from 
the ventral surface of the body, except at one point, where a narrow vitelline stalk still 
connects the two. Lereboullet speaks of such a pedicle in Esox (No. 95, p. 612); but 
this has not been confirmed, and in no case probably does the splanchnopleure surround 
the yolk, and form a narrow pedicel, until the latter has diminished to a very large extent. 

Ejpiblast. — The external epiblast undergoes little change. We have seen that it is 
established as a single layer of cells, which very early become flattened and in section 
spindle-shaped. They form, in fact, an epidermis or corneous stratum, ep; but are not for 
some time marked off with any distinctness from the lower-layer cells of the blastoderm.* 
In the region of the head they first show their characteristic features ; but they retain 
their primary rounded, polygonal outline at the posterior extremity of the embryo till 
much later (PI. IV. fig. 5c). These last-named cells, as remarked on a prior page, are 
not differentiated fully from the cells beneath until the closure of the blastopore. While 
over the trunk, and the area of the blastoderm beyond, the corneous epiblast extends as 
a single stratum of squamous cells, yet it may often show slight proliferation, and 
present more than one layer. In section, through the head of an early flounder 
(PI. IV. fig. 3), this is so, though it is true the protrusion of the optic vesicles may 
have cut off a thin superior stratum of neurodermal epiblast. Over the blastoderm 
generally a single layer of corneous epiblast seems to be present ; the nervous layer, on 
the contrary, is many-layered, and in the middle line becomes so dense as to form 

* In some Teleosteans this distinction would seem to be well marked, for Kowalewskt speaks of it as dis- 
tinguishable soon after cleavage is ended in Carassius, Polyacanthus, and Gobius (op. cit, 1886). 


the thickened carina, ne, which presses upon the yolk (vide PL IV. fig 4). Very often 
it is so distinctly separated from the epidermal cells above, that a fissure intervenes, 
forming at the sides quite a spacious interstice (PI. III. fig. 2). The cells of the neurochord 
are full and rounded (ne, PL IV. fig. 4), but as downward proliferation proceeds, those 
forming its lateral boundary become columnar, and unmistakably mark off the neuro- 
chord from the adjacent cells, especially in the fore part of the trunk. In this region the 
cells so rapidly proliferate ventrally and laterally, that they come into direct contact with 
the limiting hypoblast, hy, below and at the sides, or at most permit a mere trace of 
mesoblast to find a place there. Further back (PL IV. figs. 5a and 56) the mesoblastic 
plates, mes, lie upon each side, and its ventral ridge alone touches the hypoblast, hy, while 
above it is limited only by the flattened stratum of epiblast, ep. Both layers of epiblast 
seem to extend over the blastoderm, and form the outer stratum of the yolk-sac, while 
below lies the extended hypoblast, which rests directly upon the periblastic cortex of the 
yolk. It is below the second epiblastic layer, which here assumes the character of a loose 
mucosa, a rete Malpighii — or rather in the lowest stratrum of this mucous layer, that the 
pigment occurs as amorphous bodies which extend over the surface of the yolk. In 
P. platessa and other forms, in which the epiblast lies immediately upon the periblast, 
the hypoblast being apparently absent, the pigment may send processes into the yolk- 
cortex ; indeed pigment may develop in the periblast itself as described on a subsequent 

Notochord. — In the earliest sections of the trunk, no trace of the notochord is seen, 
the neurochord, ne, being limited below by the single layer of hypoblast, hy, and having 
the thick mesoblast, mes, upon each side (PL IV. figs. 5a-5d). About the time that the 
lip of the blastopore has reached the equator, a median mass of cells (nc, PL III. fig. 11) 
intervenes between the keel of the neurochord, nee, and the hypoblastic stratum, hyp. 
These (notochordal) cells are rounded, and rapidly show a somewhat concentric 
arrangement, quite unlike the depressed cells of the stratified neurochord above 
(PL III. fig. 11; PL IV. fig. 56). The notochordal cells, nc, it is true, are not 
separated by any definite line of demarcation from the ventral ridge of the neurochord, 
ne ; but as the cells of the latter are unmistakably squeezed upwards by the pressure 
of the notochord below, this could hardly happen were the cells of the notochord a 
downward proliferation of neurochordal cells. The ventral ridge of the neurochord is 
evidently indented and its cells greatly flattened by these axial cells below. In such a 
section of the early notochord as shown in PL III. fig. 11, the possibility remains that this 
axial rod of cells is a remnant of median mesoblast, left when the lateral mesoblastic plates 
are sundered as protovertebraj, but the difficulty of such derivation lies in the fact that 
the mesoblast never appears to be confluent in this region ; on the contrary, when once 
the notochord is indicated, it is sharply marked off from the mesoblast on either side. 
Thus in the section (PL IV. fig. 10), while the notochordal mass, nc, is not clearly 
separated from the hypoblast, hy, below, or from the epiblast (neurochord), ne, above, a 
very distinct line of division passes between it and the lateral mesoblastic plates, mes, 


though in another section of the same date (PL III. fig. 13) the notochord and mesoblast 
are not distinctly separated.* In the chick the early notochord is continuous laterally 
both with the mesoblast and hypoblast (No. 19, p. 185) ; while Van Bambeke, in agree- 
ment with Oellacher, decides from his sections that the notochord is directly mesoblastic 
(No. 114 ; also see his fig. 14, pi. iv.). A comparison of a large number of sections shows 
that the mesoblast, mes, is very clearly separated as two lateral plates, e.g., as in PL IV. 
fig. 5a ; but the notochord, even when detached from the hypoblast, and apparently in 
intimate connection with the epiblast (neurochord), is never united to the myotomes. 
The hypoblast, hy, it is noteworthy, is hardly distinguishable in this region, as though it 
had been almost entirely used up in the formation of the notochord, for at the sides it is 
well-defined. Balfour noticed a similar thinning out of the hypoblast, and he states 
that only by high powers could the continuity of the stratum be made out (No. 14, 
p. 683). Van Bambeke again denies that the hypoblast exists here at all, affirming that 
the notochord is at first in direct contact with the periblast below (No. 20a, fig. 15, pi. 
ii.), a layer of cells being afterwards pushed in from each side, and thus separating the 
notochord from the cortex of the yolk. The character of the cells, on close examination, 
shows the distinguishing features insisted on earlier, viz., the (dorso-ventrally) depressed 
condition of the neurochordal cells, ne, and their arched stratified disposition ; whereas 
those of the notochord do not exhibit these features, and the contrast is still more 
emphatic at a later stage. In addition to their rotund condition, the notochordal cells 
are seen in longitudinal section to have a transverse arrangement, such as would be pro- 
duced by an antero-posterior pressure (nc, PL IV. fig. 15), and this is interesting as indi- 
cating, what we have already suggested was possible (see p. 729), viz., that the notochord 
may be pushed forward to a certain extent from the primitive streak. 

Unlike the condition in Elasmobranchs, the notochord of Teleosteans is at first clearly 
differentiated in the mid-trunk or mesenteric region (PL XXII. fig. 12, nc), and gradually 
extends forward, ending indefinitely above the middle of the cardiac rudiment, as in Molva 
vulgaris, on the first or second day (PL V. fig. 8). It curves downward, and sometimes 
seems to turn slightly to the left, as in T. gurnardus, on the ninth day. A section through 
the otocystic region (PL IV. fig. 4) shows a mere trace of median hypoblastic proliferation, 
while in the post-mesenteric region the activity of the hypoblastic cells has resulted in the 
formation, not of a distinct notochord, but of an arch of columnar enteric cells bridging 
over a cavity (PL IV. fig. 56), suggesting a condition identical with that represented in 
Balfour's figure of this region in Petromyzon (No. 11, fig. 39, p. 86), in an Elasmobranch 
(No. 15, fig. 1, c, pi. xxix.), and in Lacerta (No. 14, figs. 2, 3, pi. xix.) ; while Scott and 
Osborn's figure of Triton (No. 147, fig. 5, pi. xx.) no less closely resembles it. The 
last-named observers clearly saw that the notochord originated from the upper wall 
of the alimentary canal, as is indicated in their figure just mentioned. The outgrowth of 
the notochord from this enteric roof, figured in PL IV. fig. 5b, is not actually seen, but 

* In Lepidosteus, Balfour and Parker noticed a similar sharp separation from the mesoblastic plates, while the 
hypoblast had more intimate relation to the notochord, but they could not decide as to its real origin. 


it is demonstrated that where the hypoblastic cells are not converted into the enteric arch 
they proliferate to form the notochord anteriorly ; while the notochordal cells, nc, origin- 
ating in this way merge posteriorly in the enteric roof (PI. IV. fig. 5c), precisely as they 
unite in Elasmobranchs (No. 15, p. 683). 

The notochord arises then as a ridge, or median proliferation, of the hypoblast in the 
posterior portion of the mid-trunk ; extending from that region, anteriorly, chiefly by a 
progressive proliferation of hypoblast below, but doubtless to some extent, as already 
mentioned, by a forward pressure of the hind part of the rod which is first formed. 
Kolliker's view, that the notochord is continuous with the primitive streak (No. 81), 
from which latter mass of cells the mesoblast arises and progresses forward, is consonant 
with such a forward growth of the notochord in Teleosteans as we have indicated. 
While the mesoblastic origin of the notochord is not generally accepted, there remains a 
possible mode of origin which sections do not directly discountenance. If it is neither 
formed from mesoblast nor hypoblast, it may yet be an axial differentiation of lower- 
layer cells, constituting in situ a median rod, when the mesoblast plates are cut off 
laterally, and the neurochord is defined above. Such a derivation has much in its favour, 
if we consider such sections as are given in PL III. figs. 2 and 11, and it is the conclusion 
adopted by Balfour and Deighton. In the case of the chick they found a median plate 
of cells, not as yet divided into mesoblast or hypoblast, together with a short column of 
cells originating from the primitive streak (No. 19, p. 186), and these form the notochord. 
In Cyclostomes (Petromyzon) the notochord is formed by a vertical reduplication of axial 
hypoblast-cells, as Calberla (No. 39) showed, and as Balfour confirmed (vide No. 11, 
p. 87, figs. 39, 40) ; but whether this holds true for the Elasmobranchs, or whether axial- 
layer cells, as above stated for the chick, form it, Balfour found himself unable to decide. 
This uncertainty in regard to the origin of the notochord is further shown by the fact 
that Rudwaner was of opinion that it arose from the epiblast ; while Kingsley and Conn 
considered it hypoblastic, as also did Calberla for Petromyzon, Syngnathus, and Rana. 
Braun, again, held that in the parrots the notochord was mesoblastic. 

In Teleosteans Kupffer affirms the origin of the notochord to be one of the unsolved 
problems of embryology, and he declines to come to a decision on the question (No. 87, 
p. 222). We have pointed out, however, that its hypoblastic origin is most in accordance 
with the sections. The large cells (nc, PI. IV. fig. 56) above the primitive enteron, 
there is little doubt, are the first traces of the notochord, which further forward is already 
partially defined. Fig. 11, PL III., again, is most satisfactorily interpreted as demon- 
strating the meeting of cells from above (the neurochordal proliferation) with the noto- 
chordal cells (hypoblastic proliferation) below. The cells of this longitudinal rod, nc, present 
much the same features as the adjacent cells, mes and hyp, though the neurochordal cells, 
ne, above always exhibit a more or less depressed appearance. At its anterior end the 
notochord grows rapidly forward, and, as Scott found in Petromyzon, it extends beyond 
the hypoblast of the alimentary canal into the cephalic region. There is, in fact, an 
anterior proliferation of notochordal cells (No. 146, p. 145). We see that in a series of 


sections (PI. IV. figs, bb-bd) through the posterior region during the early stages of the 
notochord nc, it widens out and becomes lost in the primitive streak (prs), or rather 
merges in the upper wall of the gut, both disappearing in the caudal mass of indifferent 
cells (prs, PL III. figs. 3 and 12), just as in Lepidosteus the notochord is not separated 
from the lateral mesoblast, nor the latter from the neurochord, posteriorly. When ulti- 
mately it is defined, and extends from its hind emargination to its oral termination 
(nc, PI. V. fig. 4), its cells do not long retain their primitive condition. They are not, as 
in Triton, primarily large cells which divide into small cells, and again break up to form 
larger cells once more (No. 147, p. 467), but are cells of small diameter — agreeing with 
such as for the most part compose the embryonic trunk, and become larger by an increase 
of their substance. Thus in a haddock of the fourth day (nc, PL III. fig. 13) with the rim 
at the equator, they can only with difficulty be distinguished from the mesoblast-cells, 
mes, on each side; yet when the blastopore is just closing (fifth day, PL IV. fig. 10), 
these cells, nc, are conspicuous for their large size and rounded contour, while their tend- 
ency to assume a radial arrangement is marked. The larger size of the cells in transverse 
section must, no doubt, in some measure be due to the forward pressure mentioned on a 
previous page, for only three or four cells reach across the diameter of the notochord. In 
their smaller, earlier condition six to eight cells extend across the same diameter. 

While the notochord is well defined posteriorly (nc, PL IV. fig. 10), save at its 
extreme aboral end (and Balfour and Parker found a similar obliteration of the notochord 
posteriorly in Lepidosteus)* anteriorly it is even more distinctly marked (nc, PL IV. fig. 
11), though as yet no chordal membrane surrounds it. When the blastopore is closing 
the notochord does not reach as far as the pectoral region, but on the first or second 
day afterwards it extends quite to the point where the cardiac swelling appears (PL V. 
fig. 8). About the time that the lenses of the developing eyes are visible the oral end 
of the notochord is sufficiently well marked to exhibit the characteristic flexure in 
front of the heart ; but at its aboral end it spreads out slightly, and vaguely terminates 
in the tail which is now defined and prominent (nc, PL XXIII. fig. 9 ; PL V. figs. 8 
and 10). Transverse striations soon cross the notochord, due to the continued for- 
ward pressure of its cells from behind, and cells here and there are seen breaking 
down, so that discoidal plates, or rather irregular vertical septa separating intra- 
cellular chambers, are formed. From its oral to its aboral end a continuous series of 
these chambers appears, resembling the " interrupted pith " of botanists (nc, PL IV. fig. 
12). The process of vacuolation, of the breaking down, and aggregation of flattened 
cells in serial fashion, is preceded by the assumption of a radial arrangement in the cells 
about to suffer alteration, their nuclei showing a centripetal movement, so that they are 
mainly found along the central line of the notochord (nc, PL IV. fig. 10, just as 
Oellacher represents in No. 114, Taf. iv. fig. xvi 2 , &c). The process in Clupea, according 
to Kupffer, is not such as we have described for our forms, for the refractive discs, he 
states, are formed by the confluence of minute granular particles in the primary cells as 

* Phil. Trans., 1882, ii. p. 365. 


they become flattened. A simple series of these transverse divisions, termed by him 
" secondary cells," is formed, and in a longitudinal section of the notochord he figures the 
various stages (No. 87, Taf. iv. fig. 44). The irregular transverse septa in section (nc, 
PI. IV. fig. 12), are, however, evidently due to the adhesion of the walls of the primary 
notochordal cells, and the confluence of their protoplasm to form large interstices.*" 
These septa become still more desiccated, and form a fine but complex meshwork, the 
outermost portions of which constitute a limiting membrane. No such investment as the 
latter as yet exists, though at a very early stage in Elasmobranchs Balfour made out a 
special sheath, in fact, very soon after its formation (No. 11, p. 684). In Teleosteans 
the neurochord above, and the hypoblast beneath, are in direct contact with the 
constituent cells of the notochord during the early metamorphosis just described. A 
stratum of flattened mesoblastic cells, it is true, at so early a stage as fig. 12, PL IV., may 
clothe the sides of the chorda, nc, while a thickened layer of similar cells may intrude 
between it and the hypoblastic enteron, g, destined, no doubt, to contribute to the later 
perichordal sheath. This external mesoblast is probably the special sheath described by 
Lereboullet at an early phase in Esox (No. 93, p. 527) ; but at this stage the mass of 
cells is external to and independent of the notochord, which must be regarded as a naked 
cord of cells undergoing rapid vacuolation. 

When vacuolation has proceeded so far that the mere transverse fissures of fig. 12, 
PI. IV., become converted into the spacious chambers more or less rounded, especially 
in the caudal region (PI. XV. fig. 4, nc), and subsequently into the more irregular 
spaces (PI. XL fig. 11, and PI. XV. fig. 7, nc), those collapsed cells which are not 
included in the septa will be pushed outwards, and form, as in fact they do, a continuous 
circumscribing sheath. The process is purely one of vacuolation, and the breaking down 
of the boundaries of smaller cells to form larger ones. No dot-like aggregations, such as 
Kupffer describes, seem to take part in the process, nor do scattered yolk-spherules 
(Balfour, No. 11, p. 684) or oily elements occur (Lereboullet, No. 93, p. 527) in the con- 
tents of the notochordal cells. The contents of the cells are fluid, clear, and homogeneous, 
and often exhibit a slightly pinkish tint in certain lights, as in T. gurnardus on the fifth 
day. Lereboullet did not notice in his forms the early condition — the primary cells of 
the notochord, for it was already transversely striated when he first observed its 
structure ; and he notes the remarkable feature, just referred to, that through all its 
substance oily elements are dispersed (No. 93, p. 527, pi. ii. fig. 44). With its increase 
in length the notochord grows in diameter, a condition which is precisely the opposite of 
that described by Scott and Osborn in Triton, for in that form the notochord is largest 
in cross-section during its earliest stages, and greatly diminishes in diameter during sub- 
sequent stages (No. 147, p. 467). The increase in diameter of the Teleostean notochord 
stretches the cells of the sheath, i.e., the superficial cells of the chorda ; thus they become 

* These interstices, with fine membranous limits, form a series of discs placed one behind the other along the 
whole length of the chorda. They form large discoidal cells, which at many points do not entirely pass across the 
notochord, as they vary considerably in diameter. 


very thin, and at times almost imperceptible (nc, PL VII. fig. 6). The true notochordal 
sheath during the later larval periods is very delicate and fine (PI. VII. figs. 6, 6a), nor 
does it, as Balfour indicates (No. 11, p. 546), ever become thicker or more definite. It 
is nucleated (n, PI. XV. fig. 7), as Gegenbaur showed in the greatly thickened sheath 
of Salmo salar ;* but the nuclei are irregularly arranged, and in some sections they 
are so sparse as to suggest the presence of an enucleate stratum (cs, PI. XV. fig. 7), 
though this condition is easily explained by its mode of origin. In horizontal sections 
of the chorda the flattened cells of the sheath overlap and produce a more or less regular 
tessellated appearance (Pi. XV. fig. 7). Kupffer says that this sheath in the herring is 
homogeneous and without nuclei (No. 87, p. 222), while he describes a round nucleus in 
each chamber of the vacuolated notochord. Such nuclei in our forms are rare, though 
sections often pass through the points, where several septa unite and produce the same 
appearance as nuclei in the septa would do, but they are simply sections of the junction 
of cell-wells, or, at times, merely the collapsed contents of the notochordal chambers. 

Of the subnotochordal rod, which has been described by Balfour, Oellacher, Ryder, 
and others, nothing definite can be here stated. It would, in fact, not appear to be 
developed in the forms specially considered in this paper, though Ryder mentions it in 
Alosa and Salmo as a well-marked strand of cells ; and Oellacher is of opinion that it 
shares in the development of the aorta along the under surface of the chorda dorsalis. 
The intruding mesobiast limiting the chorda below in the Gadoids, gurnard, and others 
is an indefinite lamella figured in its earliest condition in PI. IV. figs. 12 and 18, which 
subsequently forms a median meshwork in which the early haemal lacunae are developed, 
while laterally the renal connective and other tissues are formed out of it [vide PI. VII. 
figs. 1, 4, 6).t 

Vertebral Column. — The vertebral column and its costal appendages belong as such 
to a stage subsequent to the larval condition proper, and, in this place, little more 
can be done than simply to touch upon certain points observed before the close of the 
first month after extrusion. 

The cod and haddock will be mainly referred to, as the condition of the vertebral 
column shows great differences in various Teleosteans ; in some forms cartilage-cells 
appearing, and cartilaginous arches developing soon after hatching, whereas in others 
no such elements are present until the embryo is about a month old. Lereboullet, 
indeed, was unable to make out any ossification in the perichordal sheath, in Perca, until 
the young fish was three months old (No. 93, p. 644). 

The condition of the notochord before and after hatching has been described, 
and sections of Q. morrhua or G. aglejinus, on the seventeenth to the twentieth day, 
show the same simple structure almost unchanged — the cuticular layer or nucleated chordal 

* Comp. Anat., Lond. 1878, fig. 2216, p. 427. 

t The myotomes are broken up into fibres about the ninth day (two days before hatching in P. flesus). Eight or 
ten of these fibres, in horizontal section, are seen passing across the shorter axis of the myotome, which is rectangular, 
and measures about - 001 in. x "0018 in., the longer measurement including the columnar external stratum of cells 
lying beneath the epiblast. 

VOL. XXXV. PART III. (NO. 19). 6 C 


sheath proper (cs, PI. XL figs. 14, 15) being very thin, and the mesoblastic perichordal 
sheath (pes) but little increased in thickness. This latter sheath, of protovertebral 
origin, is equivalent to the skeletogenous layer of Plagiostomes, though in them it is 
greatly thickened. In this perichordal sheath an outer lamina can be made out, especially 
when the rudiments of the vertebral bodies and arches are developed, as it forms their 
outer investment. Below the sheath and its elastica externa, a layer of cells in the sharks 
intrudes, coming, as Balfour thought, from the outside, and forming the cartilaginous 
tube around the chordal sheath. From this intruding layer the future vertebrae are 
formed, and it may be termed the inner skeletogenous layer : it is the inner half of the 
skeletogenous tube. Outside the membrana elastica externa, however, another meso- 
blastic layer is formed, viz., the outer half or outer skeletogenous layer from which the 
neural and haemal arches are developed. In the less primitive sharks, such as Mustelus, 
the Kays, and others, the inner skeletogenous layer is much reduced, and the elastica 
externa is considerably nearer the chorda than in Cestracion and Notidanus. If we 
consider the process of reduction to have affected the external portion until no outer half 
exists, we can then look upon the perichordal sheath in Teleosteans as the inner half of 
the skeletogenous layer, reduced, but still bounded by its outer limiting layer, viz., the 
membrana elastica externa [met, PI. XL figs. 14, 15). There is, of course, difficulty 
in separating the parts of a sheath so thin as that surrounding the notochord in 
Teleosteans, but in a few forms, e.g., Cyclopterus, in which cartilage develops somewhat 
precociously in the vertebral column, large chondral cells appear in this external layer, 
which passes upward, and over the. spinal cord as a membrana reuniens superior. The 
cells likewise ascend up each side of the cord, forming the rami of the neural arch. 
Similarly the ventral arch is developed. In many forms, however, the arches and outer 
osseous laminae of the vertebral bodies are not preceded by preformed cartilage. In such 
cases (e.g., Gastrosteus) the osseous matter is clear, homogeneous, and brittle (Pouchet's 
" spicular substance," Kolliker's "osteoid matter"), and exactly resembles in its chitinous 
appearance the clavicular portion of the pectoral girdle, and the maxillary elements of the 
upper jaw. The presence or absence of this spicular substance seemed to Kolliker of 
diagnostic value for classificatory jDurposes, but as Pouchet points out (No. 119, p. 274), 
both spicular substance and osteoplastic tissue may occur in the same form. Pouchet 
states, and seems to be the first to do so, that in some cases osteoid processes, and in 
other cases cartilage, with osteoplasts, form the superior and inferior vertebral arches. 
But whether arising as bars of regularly disposed chondroplasts, or as homogeneous 
spicular deposits, the vertebral bodies, and their projecting dorsal and ventral rami, are the 
products of the perichordal sheath, and arise within its definite limiting layer. The view 
that the main part of the sheath in Teleosteans is a thickened membrana elastica interna, 
and derived from the cells of the chorda itself, is not supported by sections, inasmuch as 
the hypoblastic notochordal sheath always remains extremely thin, and even when 
well developed, as in the Salmonoids, is still merely a single stratum of flattened cells. 
In Elasmobranchs W. Muller recognised an elastica interna closely investing the 


chordal sheath, and Balfour refers to both layers as closely adherent, though distinct, but 
the former apparently decreases in thickness, and is then difficult to see (No. 15, vol. xi. 
p. 421). From the mesoblastic perichordal sheath alone the vertebral bodies originate, 
while its outer limiting stratum (the elastica externa) gives origin to the arches. The 
neural arches precede the haemal in development ; no trace, in fact, of the ventral 
processes being discernible when the neural arches project some distance dorsally. 

Of course, in a degenerate skeletogenous layer, such as the Teleostean perichordal 
sheath, the identification of the precise layers, seen more favourably in other fishes, is 
attended with much difficulty ; and one of us, in attempting to distinguish the different 
laminae, has referred to the outer layer as a " limitans externa " (No. 122, p. 454) ; indeed, 
the opinion expressed that the existence of an " elastica externa " in Teleosts, is a doubt- 
ful point, is supported by the fact that such a membrane does not properly exist in 
Amphibians, as well as in the Amniota. Favourable sections of Teleostean embryos, 
especially such a form as Cyclopterus, bear out, however, the above interpretation, the 
external layer being very distinct. Outside the perichordal sheath itself in post-embryonic 
stages plates of spicular substance develop. Thus in a young but mature specimen of 
Pleuronectes, the oral end of the notochord is seen to have acquired such a spicular 
sheath — formed apparently in the connective tissue outside the external limiting 
membrane — a distinct interspace separating the plate from the perichordal sheath. Four 
rami of the same chitinous substance project, one pair dorsally and one pair ventrally, 
and are well seen in sections through the otocystic region. 

Branchial System. — The head of the Teleostean embryo consists, as already indi- 
cated, of an expanded mass, chiefly neurochordal, or rather brain-tissue, and separated 
from the cortex of the yolk below by a thin layer of hypoblast (hy, PI. III. fig. 1). The 
hypoblast forms here the roof of the sub-oral cavity, which has no floor, or rather, its floor 
is simply the periblast enveloping the yolk. Behind and below the ears a large oval area 
is apparently pushed in, resulting in the perforation of the lateral epiblast on each side of 
the otocystic region, these fenestrae (poa) communicating with the primitive mouth- 
chamber within (PI. VIII. figs. 3, 4). This opening, which may be called a primitive 
opercular opening (poa), though the true operculum is a new and later growth, is plairly 
visible in Molva vulgaris on the fourth day, along with a number of superficial irregu- 
larities, doubtless connected with the active changes going on at this point in connection 
with the branchial arches (PI. X. fig. 6). The significance and function of this cleft 
(Spritzloch) upon each side is not readily understood, as the oesophageal lumen is not 
apparently open in front, and any perivitelline fluid which gains access to the sub-cephalic 
chamber, probably cannot find passage into the alimentary canal. Hoffman, however, 
speaks of it as produced by an evagination of the oesophagus, at first below the otocyst, 
but shifting forward and opening in front of the ear (No. 69, p. 7 ; vide his pi. i. fig. 5, 
emb. sp., also fig. 3, on p. 7). These embryonic " Spritzlocher," he says, are merely 
transient structures, and the ' interesting question is raised as to whether they may be a 
reminiscence of the outer or extra-branchial system of the Cyclostomes, of which traces 


are observable in the Elasmobranchs. Each aperture, poa, has a strongly marked 
corrugated border or fold, which sweeps in a graceful curve round the opening, and 
passes forward beneath the otocysts (au, PI. VIII. fig. 4), for in pelagic forms, the shifting 
of which Hoffman speaks was not observed, and in front the fold is gradually lost. 
The opercular flap is a much later outgrowth from the tympanic region, apparently a 
fold of the integument, which protrudes, and grows backward over the gill-slits (ope, 
PI. XL figs. 10, 11). Below the hind-brain and otocysts, the hypoblast shows great 
increase in its cells, so that by the time the heart is defined it forms a thick supra-cardial 
plate (PI. XI. figs. 2, 7, 8), beneath which mesoblastic cells make their way by a down- 
ward growth of the lateral cephalic masses. The sub-cephalic floor of hypoblast and 
mesoblast is limited below by a somewhat ill-defined layer of nucleated periblast (per, 
PI. XI. fig. 8). The mesoblast thus intruding into the oral hypoblast becomes columnar, 
and forms paired rod-like masses (PI. XL figs. 5, 6, 7). The cells are concentrically 
arranged along the axis of the transverse bars. Lereboullet evidently refers to the down- 
ward growth of mesoblast, and speaks of it as a ventral lamella (i.e., splanchnopleure), out 
of which, he adds, is formed later "the maxillary and hyoidean elements, and the gill- 
supports." While the appearance of serial mesoblastic thickenings along the floor of the 
pharynx is a marked feature in Teleosteans some days before emerging from the egg, 
their disposition and conformation are very difficult to make out. There is indeed con- 
siderable variation in the condition of the branchial region, and this is especially seen in 
newly-hatched gurnards. Usually three branchial bars are visible (PI. VIII. fig. 8) as pale 
structureless bands, with intervening cellular tissue, and passing transversely towards the 
mesial ventral line beneath the otocysts. Balfour and Parker (No. 18) noticed in 
Lepidosteus, six days after fertilisation, two transverse streaks on either side of the hind- 
brain. From a comparison with the sturgeon they judged them to be branchial clefts, but 
in section these clefts could not be detected. In the early condition of the branchial 
system the study of sections is by no means easy. C. Vogt shows, in an embryo of 
Coregonus palcea, thirty-six days old, branchial vessels, but indicates no skeletal bars (vide 
No. 155, Taf. ii. fig. 58). The fact seems to be that, soon after the arches are distinctly 
formed as definite bars, a vessel, or rather a long thread-like lacuna, is formed along the 
posterior margin of each bar (PL XL figs. 9, 11). Five transverse bands, some- 
times an indication of a sixth, extend later on each side across the floor of the wide and 
flattened oesophagus, from a point just behind the eye to a little distance beyond the 
otocyst, so that the floor becomes raised into a series of cross-ridges which cease in the 
middle line, and between the ridges the hypoblast is pushed so that the mesoblastic 
ridges gradually become separated by hypoblastic septa. Parker speaks of these ridges 
as separated by the dehiscence of the thinned interspaces between them (No. 117), but 
this hardly describes the process correctly, the rib-like thickenings being more truly 
separated by the paired hypoblastic diverticula or septa, these being pushed out from the 
sides and floor of the pharynx and affecting the differentiation of the serial gill-arches. 
Dehiscence takes place, it is true, but much later, and it results in the formation of 


actual slits, a phenomenon not seen until long after the arches are fully differentiated. 
From this protrusion of enteric hypoblast, Sedgwick likened these paired pouches to 
nephridia ; indeed, he considers them homologous, the kidney-system of vertebrates never 
overlapping them, but commencing behind their posterior limits (No. 148, p. 67). The 
clefts or gill-openings are probably not formed until some time after extrusion from the 
egg, but the hypoblastic diverticula indicate their future position, and the dense meso- 
blastic masses between them form the branchial skeleton or gill-arches. The latter in 
their early condition appear as a series of rounded or subquadrate structures, when seen 
from below (PI. VIII. fig. 5), but viewed from the side fine striations merely are observed 
passing dorso-ventrally with a slight inclination forward, these striations being the linear 
outpushings of the oral hypoblast (PI. VIII. fig. 6). The arches thus early indicated are 
not simultaneous, and Leeeboullet observed in the embryo of Perca that they appeared 
successively from behind forward (No. 93, p. 616). The precise stage when the branchial 
clefts are open cannot be stated. There is no doubt it is very late, for long after the 
arches are clearly defined the slits are still unformed, even in so advanced an embryo as 
Gastrosteus (PL XL fig. 9), in which the mouth is open, while the hypoblastic cells, hy, 
which pass down between and surround the bars, bra, still form a continuous layer. A 
fifth branchial arch can be made out, but remains rudimentary in the Gadoids and other 
forms here considered ; while anterior to the four branchial arches proper, two pairs of 
stout bars are developed at an early stage, viz., the hyoid (hy, PI. X. figs. 2, 3 ; 
PI. XIII. figs. 5, 6), and in front the mandibular (mn, — Meckel's cartilage). Both these 
arches undergo a more complex development than the branchial rods behind, and with 
the appearance of cartilage-cells, both are readily distinguished by their greater length 
and stoutness, as well as by their direction, both extending forward and tending rapidly 
to complete the arch on the floor of the mouth. The upper portion of the first or man- 
dibular arch becomes expanded (PI. IX. fig. 6 ; PI. XIII. fig. 5) ; and Parker speaks of 
it as well marked in the salmon (No. 117, p. 113), splitting longitudinally into two, 
giving origin in this way to a fore part, the mandible proper, and a hind portion, the 
hyoid. In our forms the hyoid is already well developed when the division of the man- 
dibular cartilage takes place, and it would appear therefore that the posterior portion, hm, 
which is the stronger, and much expanded at its upper extremity, is really the hyo- 
mandibular, thus arising as an element separate from the hyoid, while the narrower 
anterior part, pq, also split off, becomes the palato-quadrate. Before this splitting is 
complete, the extended lower part separates as the primary lower jaw or mandible, mn, 
and its proximal part becomes enlarged (PI. IX. figs. 6, 7 ; PL X. figs. 2, 3), to afford 
an articulating surface for the two suspensory elements above, the palato-quadrate and the 
hyomandibular, which separately articulate, the former doing so earlier than the latter, and 
more directly (PL IX. fig. 7). From the proximal portion of the mandible an anterior 
process grows out at a subsequent stage (PL IX. fig. 7), while in the angle below the 
end of the stout and broad hyomandibular, a small element, the angular, develops. The 
forward growth of the palato-quadrate cartilage must be a late phenomenon, for the pre- 


maxillae and maxillae develop in advanced embryos as paired translucent rods (PL XL 
fig. 20), which gracefully curve, like bars of chitin, below the eyes forward to the 
ethmoidal region, and form the sole lateral supports of the oral roof (PL X. fig. 1 ; 
PL XIII. fig. 7). They are essentially superficial, and lie in a thin stratum of membrane 
which stains deeply, called by Pouchet " tissue generateur," and occupying the situation 
of Parker's " subocular bands," though he regards them as the rudiments of the pterygo- 
palatine arch (No. 117, p. 113). The homology of these dermal maxillary rods, with the 
labial cartilages of more primitive forms, as suggested by Dr Gunther (No. 61, p. 90), 
is of much interest. A pair of curved bars, probably palatine elements, are also developed 
in the roof of the oral chamber at a late embryonic stage. They are irregular in thick- 
ness, slightly curved, and attenuated at the extremities (PL XL fig. 18). 

When first distinguishable, the pharyngeal bars consist simply of solid mesoblastic 
thickenings passing along the lateral and ventral walls of the mouth, and more or less 
oblique in direction ; moreover, in cross-section, these thickenings are found to be paired, 
and united in the middle line, forming a roof over the pericardial chamber (PL XL 
figs. 1-3, 6-8). The cells assume a columnar arrangement, and constitute laminae, 
which appear as parallel superposed strata, when the bar is cut longitudinally (PL XL 
fig. 9), but in a cross-section of a bar these strata are observed to be somewhat concentric 
and laminated (PL XL figs. 6-8). Each rudiment of a branchial arch (fg, PL XL 
figs. 6-8), when fairly defined, consists of a cylindrical mass of cells, concentrically 
arranged round the central point of the bar, and limited above by the epithelial hypoblast 
of the pharynx, and below by the pericardial hypoblast. They increase in length, and 
change from the transverse to the antero-posterior oblique position (PL VIII. fig. 9), the 
inner extremity of each pair of arches apparently shifting forward, so that they point 
anteriorly (PL X. figs. 2, 3, 5); while their upper and posterior parts, which extend 
up the lateral walls of the pharynx, have moved very slightly from their primary position. 
Neither the mandibular nor the hyoidean arches are so markedly transverse in situation as 
the branchial bars proper, and they alter very little in position as development proceeds. 
In the gurnard, three days old, at the anterior end of the hyoid arches, i.e., where the 
copula is formed, a large boss occurs, formed chiefly by a free development of the lining 
membrane of the oral floor. This membranous expansion (really a lingual rudiment) pro- 
jects as a large irregular elevation on the floor of the mouth, and is lifted up by the 
erratic movements of the hyoid arch, as though the operation of deglutition were being 
performed (PL XIV. fig. 2). Gradually the arches lose their dense indifferent appear- 
ance, and become converted into cartilage, the small primary cells being broken down, so 
that each bar consists of larger flattened elements placed transversely, and giving the 
arches a transversely striated appearance (PL IX. fig. 5). The flattened cells become 
hyaline, and each arch shows a single column of hyaline discs contained in a thin peri- 
chondrial membrane. The first two arches wholly assume this character, and are seen to 
be composed of these discs or chondroplasts placed one above the other along the 
whole length of each bar (see the figure just referred to) ; but in the four arches which 


follow, only a portion undergoes this change, viz., that part of each bar nearest the 
pharyngeal cavity, i.e., throughout the entire upper part of each. The rest of the bar 
remains indifferent in structure until a tubular cavity is formed from end to end. This 
tube at first is apparently single, but later is divided by a delicate septum into two tubes, 
an upper arterial and a lower venous trunk. External to and below the haemal canals 
the loose epithelial covering of the bar becomes nodulate, a double row of papillae pro- 
jecting on the posterior and ventral side of each arch. The appearance of these gill- 
rudiments is thus preceded by a considerable interval by the conversion of the arches 
into cartilage, as Lereboullet observed in Perca (No. 93, p. 623), the same author noting 
in that species the growth of the gill-tubercles from the soft cellular membrane covering 
the gill-arches (see his plate iii. fig. 7). Moreover, he speaks of them as hollow (p. 627), 
an appearance probably due to the intrusion of mesoblast into each papilla, which is thus 
provided with a mesoblastic core and a hypoblastic epithelial covering. 

The formation of these branchial tubercles belongs, it may almost be said, to the first 
post-larval stages, and their subsequent development into the branchial fringes of the 
adult leads beyond the present limits.* 

The further development of the early cartilages may be easily followed in a large 
chondral element such as the hyomandibular, or the massive mandible itself. The disc- 
like chondroplasts which form a single column along the entire length of the bar (PI. IX. 
fig. 5), slightly alter in form, becoming wedge-shaped when seen laterally, and lie over 
each other in an alternate manner, as though about to separate into two rows, — sometimes, 
indeed, a disc becomes thin in its median part, and divides, resulting in two wedge- 
shaped chondroplasts. Thus the original single column of chondroplasts becomes broader, 
and exhibits two or more rows (PI. IX. fig. 7). In the mandible this change affects the 
upper or articular portion, while the anterior growing part, which continues to lengthen 
until the cartilages of each side meet at the tip of the extended oral floor, still maintains 
its simple columnar character, and consists of a single series of chondroplasts. In other 
cases, as Pouchet noted (No. 119, p. 296), the chondroplasts towards the extremities 
lose their disc-like form — becoming irregular in outline and mingling with the enveloping 
tissue — just as in the limbs of young Amphibians. In the region of the joints, as in the 
upper or articular portion of the mandible, the chondroplasts become irregular, numerous, 
and disposed round the joint conformably to the contour of the articulating element 
(PI. IX. fig. 7). The development of a crest is due to the protrusion of similar small 
irregular chondroplasts, which grow out, row upon row, as a strong lamella. Pouchet 
regards these new chondroplasts as not due so much to fission of existing chondro- 
plasts as to development from the nuclei, so plentiful in the perichondrium or " tissu 
generateur" which clothes the bars (No. 119, p. 298). These nuclei he describes as 
crowded at the margin, and, as they pass inward, become separated by the intrusion of 
the hyaline matrix. 

* In Esoz and Perca the yolk has decreased and its circulation has heen almost obliterated, according to 
Lereboullet, when the gills are formed (No. 93, pp. 613, 627). 


In the young cod, three weeks after hatching, the branchial system is wholly converted 
into cartilage, and forms a complex series of translucent hyaline bars, in which the four 
parts — the epi-, cerato-, hypo-, and basi-branchial pieces can be distinguished, and the 
small rod-like azygos pieces in the middle of the oral floor form the several copulse for the 
respective arches. 

In T. gurnardus, and other pelagic forms, the cartilages of the jaws apparently 
become stiff and immobile about the eighth day after hatching, and the hyoidean 
apparatus also shows no regular movements. The fish, however, by its forward jerking 
motion, drives water into the widely-open mouth, and aeration is thus easily effected, for 
the opercular opening is broad, and the operculum itself projects outward and backward, 
as a thick flap of the integument. 

The mandibular rami, mn, continue to lengthen upon each side to such a degree that 
they project much beyond the upper jaw, and a symphysis is formed at the anterior margin 
(PI. X. figs. 1-5). No feature is more striking than this extraordinary development 
of the lower jaw, and in sickly and abnormal embryos it produces the most fantastic 
appearances — the protruding mandibles frequently curving downward, so that the gape 
of the young fish is remarkably wide (PL XIV. fig. 2), and even in normal examples this 
extension of the floor of the mouth, and the mobility of the lingual and hyoidean struc- 
tures, increase the oral aperture very much (PL X. figs. 1-3, 5, 5a), and contribute 
doubtless to facilitate the capture of the minute organisms which form the earliest food 
of the young Teleostean. 

Skull. — The capsule enclosing the brain is, like the rest of the body of the embryo, 
simply a thin epiblastic layer composed of the flattened corneous stratum, and the thicker 
sensory remnant beneath. 

Between this ectodermal covering, ep, which though expanded in the form of 
a bulbous protective capsule, can scarcely be regarded as a cranium, and the brain-mass 
below, a space intervenes occupied by a transparent substance, apparently of a jelly-like 
consistency. This space, ss, filled with fluid, is inconsiderable during the earliest stages 
within the ovum (PL XIX. fig. 10), and even in a newly-hatched fish (ss, PL VIII. fig. 6), 
above the mid-brain, mb (optic vesicles), it is small, though larger in front (between the 
nasal capsules), fb, and behind, hb (over the cerebellum and fourth ventricle) ; but it in- 
creases at the end of the first week (PL VIII. fig. 7), and during the second or third week 
after extrusion it becomes enormously enlarged, and imparts to the more advanced embryos 
a very grotesque appearance (ss, PL XII. figs. 2, 6 ; PL XVI. figs. 1, 3, 5). Often this 
sub-epidermal enlargement abnormally develops, and embryos with the cephalic region 
remarkably swollen are not uncommon — fig. 3, PL XVI. beiDg probably such an example ; 
but under ordinary conditions the enlargement is considerable, and median sensory papillae 
appear in it, immediately beneath the corneous layer, with connecting nervous filaments 
passing downward, probably to the lateral sensory tract (sno, PL XVII. fig. l). 

At an early stage the mesoblast of the head consists of a thin stratum chiefly aggre- 
gated between the eyes and the neurochord (mes, PL IV. fig. 17), while, above, the brain 


is directly in contact with the inner surface of the epiblast. Later, however, this meso- 
blastic tissue extends and finds its way into the lateral sinuosities of the brain-surface, 
and it passes upward as a thin membrane composed of much flattened cells, which 
finally more or less completely invests the brain. The relation of the two is very 
intimate, and probably the pia mater is at this time separated, though any differentiation 
into distinct strata cannot be made out in the membranous investment (mes, PL IV. 
figs. 14, 21). From this layer, however, the three membranes — dura mater, arachnoid, and 
pia mater — are ultimately differentiated. On its inner surface pigment rapidly develops, 
as early, indeed, as the fourth day after fertilisation in some forms (p, PL IV. fig. 13). 
We have thus a double covering over the brain, for to the simple ectodermal layer (ep, 
fig. 14), which primarily covers the neurula, there is added a thickened mesoblastic 
membrane (mes), constituting the primitive membranous cranium (PL IV. fig. 21 ; 
PL XXIII. figs. 1, 2, 3a; PL XXIV. figs. 3, 5, 6). Meanwhile changes are proceeding 
at the base of the brain, and whereas it at first lay almost directly upon the yolk 
(PL IV. figs. 3, 4), separated only by a thin layer of hypoblast (hy), it now rests 
upon a floor of mesoblast (PL IV. fig. 21). This mesoblast is apparently an exten- 
sion forward of the pectoral mesoblast, which pushes anteriorly as the notochord advances, 
and when the latter finally terminates beneath the mid-brain, a plate of intruding meso- 
blast is seen extending upon each side of it and passing as a thin sheet beneath the fore- 

At the fore end of the notochord quite a dense plate is formed (PL XL fig. 2), and 
a thickened continuation of this mesoblast passes beneath the eyes, forming a projecting 
ridge of epiblast with a core of mesoblast (PL XL figs. 2, 3), which is doubtless 
Parker's " sub-ocular band" (No. 117, p. 119). These two ridges form on each side a 
lateral flap or curtain, and the head is thus raised slightly from proximity to the yolk. 
As already pointed out, before an actual oral slit appears, an oral cavity exists whose 
roof slopes considerably on each side, and meeting in the middle line forms a highly 
arched chamber. A section through this acutely angular cavity in the region of 
the posterior prosencephalon (thalamencephalon) shows the apex, so to speak, marked 
by a small and solid mass of cells, a cylindrical rod in fact, which in sections further 
forward is found to flatten out in the form of a bilobed plate, strongly suggesting the 
union and depression of two cylinders of cells. We see then at this early stage, about 
the time of hatching, that the base of the brain is strengthened by two parachordal masses, 
which lie on each side of the notochord at its oral end, e.g., in the section of Anarrhichas 
(PL XXIV. fig. 3), and form a dense basilar plate, while further forward the flattened 
parachordals cease, and in their place two thin cylinders, the trabecule, can be dis- 
tinguished (PL XXIV. figs. 5, 6), which, as just pointed out, unite and form beneath the 
thalamencephalon a single rod. # This rod again expands beneath and in front of the 

* The early appearance of the trabeculae is noteworthy when connected with the early development of the neural 
arches in the trunk. Parker's view that the trabeculae are ventral arches of the vertebral column, serially followed 
behind by the branchial arches, has been much disputed, and the early appearance of the neural arches of the vertebral 
column is opposed to Parker's view. 

VOL. XXXV. PART III. (NO. 19). 6 D 


fore-brain, to terminate in a pair of large flattened lateral horns, and an internasal plate 
centrally (PI. XI. fig. 11). These early skeletal structures are the first indications of the 
cartilaginous cranium, but as yet they are formed of closely- aggregated cells, which 
stain deeply, and on account of their density are readily distinguished from the adjacent 
mesoblastic cells out of which they have been differentiated. Whether their cells break 
down or not is difficult to make out, but they undoubtedly become antero-posteriorly 
flattened, and in cross-section the rods under consideration begin to assume a more trans- 
lucent appearance, due to the discoidal character of the constituent cells. Within a week 
or ten days after hatching, these elements are converted into clear boldly-marked nucleated 
cartilage-cells. The large parachordals as they become cartilaginous extend outward, and 
meet to coalesce with the dense cartilaginous floor of the auditory capsules (PI. VI. 
fig. 9 ; PI. XXIII. fig. 2). The trabeculse between the eyes contract, and approach the 
base of the brain in the region near the infundibulum — becoming very narrow as the 
roof of the brain expands. Further forward the trabecules, however, spread out, forming 
a large anterior plate of cartilage, slightly thinner medially, and more thickened later- 
ally, i.e., in the portion forming the cornu (PI. XL fig. 11). 

While cartilage thus abundantly develops in the skull, no trace of it is seen in the 
axial skeleton of the trunk — metameric aggregations of mesoblastic cells (Pouchet's "tissu 
generateur") alone indicating the points along the notochord where the future vertebrae 
will be formed. During this time also cartilage appears in the form of four small plates 
around each eye, all with a concave surface towards that organ, and formed of large 
cartilage-cells placed over the summit of the eye — beneath and on the anterior and 
posterior surfaces. Whether the first or supraorbital cartilage expands later to form the 
tegmen cranii, and the second to form suborbital elements, while of the remaining two 
one becomes the lachrymal, and the other or postorbital becomes alisphenoid and post- 
frontal, though probable, could not be determined from an embryo in the second or third 
week after hatching. About the middle of the third week, indeed, four series of cartilages 
may be distinguished — (1) the posterior basal, (2) the posterior lateral (auditory), (3) the 
anterior lateral (optic), and (4) the anterior basal. 

The first named constitutes the basis cranii proper (parachordal and occipital elements); 
the second includes a basal auditory plate (PI. VI. figs. 9, 10), very dense and 
massive, and affording an outer articular surface for the hyomandibular (hm), and 
probably consisting of opisthotic and pro-otic elements, as yet undifferentiated. Above 
the ear a small aggregation of cartilage-cells (epot, PI. VI. fig. 3) occurs, from which the 
epiotic and supra-occipital are probably formed, while the third series are in a condition 
too early to identify, and are best regarded simply as circumorbital cartilages developed 
at four separate centres on the surface of the sclerotic membrane. The fourth and last 
series occurring at this stage are the trabeculae, with their expanded internasal element 
and the curved lateral cornua. Into the theoretical question of the significance of these 
paired basal bars it is here unnecessary to enter. 

Of the further changes in the skull and facial elements little can be said, as at the end 


of the first month the young embryo shows little further modification. The development 
of translucent spicular plates upon the surface of the more exposed bones, especially of the 
face, is a noteworthy feature in the young fish, but belongs to the post-larval period. 
It may be noted, however, that these homogeneous spicular plates are not solely of 
dermal origin — in fact, they develop as a thin outer layer of the true cartilaginous 
elements, and arise within the nucleated perichondrium. 

The connective-tissue septa of certain muscles which are much used, become changed 
into thin rods of clear spicular substance, a median rod, for example, passing up beneath 
the pericardial cavity, and forming a fulcrum for the retractor hyoidei muscles. A similar 
bar occurs also between the genio-hyoidei. 

Brain. — The anterior enlarged portion of the neurochord, of which frequent mention 
has already been made, is really the brain. It extends the whole depth of the fore-region 
of the embryo, forming a somewhat rhomboidal mass, rounded above, deeply carinate 
below, and arched over by the epiblastic integument, while it is limited ventrally by the 
hypoblast (PL IV. figs. 3, 4). The growth of the large optic vesicles, as two massive 
ellipsoidal bodies (op, PL IV. figs. 14, 16), protruding laterally from this region, is an 
early and notable feature ; but the details will be considered later, with the sensory organs. 
The part which becomes the mid-brain (mb, PL IV. figs. 16, 17) is very early distinguished 
by its greater breadth and volume from the narrower and prominent snout (fb), while the 
hind or metencephalic part (hb) gradually passes away into the neurochord (ne) of the 
trunk. No division as yet separates the encephalic from the spinal portion of the neuro- 
chord, and the former is distinguished only by its increased breadth and depth. It is 
remarkable, too, as extending fully one-third the total length of the embryo in its early 
condition. No transverse cerebral folds appear until about four-fifths of the yolk are 
enveloped, when a cleft, very obliquely directed, appears on each side of the post-optic 
region. An anterior portion — the united mid- and fore-brain — can now be distin- 
guished from the hind-brain (hb). The latter is, very shortly after, separated by a 
similar though less marked fold from the nervous cord (ne) behind. Lereboullet 
noticed this early transverse folding, which he says is due to the brain becoming doubled 
upon itself; but he erroneously supposed that the cleft first formed is the metencephalic, 
instead of the mesencephalic, and further conceived the neural tract to consist of two 
parallel tubes. These becoming folded, produce two vertical projections, which he calls 
the cerebellar lamellae (No. 93, p. 533). It is really the cerebral fold, the cerebellum 
being, as just stated, marked off slightly later. The mid-brain, lastly, is constricted off 
by an interorbital fold, so to speak, and the three regions of the brain are now defined 
(PL IV. fig. 1 7). Reference has already been made to the dorsal or medullary groove ; 
but it is at this stage, when the brain is separable into cerebellum and united mid- and 
fore-brain, that this groove often appears in a very marked manner. Thus, in T. gurnardus, 
on the fourth day, a deep median fissure may be seen — the sides of which slope at an angle 
of about 70°. It is a temporary groove, as previously pointed out, and not apparently con- 
nected with the subsequent cerebro-spinal canal. Soon after the closure of the blastopore, 


sometimes a little earlier, a fine cleft (mc) appears by separation of the median cells of the 
encephalon along a vertical longitudinal plane. It commences in the mid-brain, and passes 
into the fore-brain, extending almost to the anterior limits of the latter (PL IV. fig. 17). 
This is the first indication of the true neural canal. It passes dorsally, ceasing before 
reaching the upper surface of the brain, and ventrally, leaving below a thick tract of 
nervous cells uncleft. The early brain thus becomes incompletely divided, as Ryder 
aptly expresses it, into "two flat thick plates of cells placed vertically between the eyes" 
(No. 141, p. 503). At its anterior termination the canal sends off two lateral vertical 
continuations, forming a cruciform fissure which marks off the fore-brain (fb, PI. IV. 
fig. 17 ; PI. VI. fig. 6) ; while in the mid-brain, as the fissure ascends, it bifurcates laterally 
and horizontally, so that the lumen of the mesencephalon, in cross section, is T-shaped 
(PI. IV. fig. 21), the roof being thinner than the walls and floor, which are very dense, a 
feature better seen in sections of Anarrhichas (PL XXIV. figs. 3, 6). No continuity of the 
central canal with the lumina of the optic vesicles seems to be completely established, and 
certainly no trace of such a connection is observed in sections at this stage. The canal 
now rapidly extends posteriorly into the trunk, and as it does so vertical lateral cavities 
are sent off, one pair in front of the cerebral fold, forming the optic ventricle or Iter 
a tertio ad quartum ventriculum, and a second pair, constituting the fourth ventricle, 
immediately posterior to the cerebellar fold (cb, PL VI. fig. 6). The most notable 
feature at this early stage is the continued lateral extension of the mesencephalon (mb), 
and its progress backward over the metencephalon (cb), until it almost covers the 
latter with its two broad lobes, which continue to increase in breadth (compare figs. 5 
and 7, PL VI.). Between the eyes we have, therefore, a prominent mesencephalic 
dome formed of two halves, narrower in front, but broad and overlapping the narrower 
posterior fore-brain (thalamencephalon) and the base of the mid-brain. Several days 
before hatching this extension of the mid-brain takes place, the T-shaped chamber 
(optic ventricle) increasing in its upper portion and its lateral regions until the roof 
above exhibits a considerable decrease in thickness and a marked columnar disposition 
of its cells. 

An embryo before hatching usually shows such a development of the mid-brain as 
above described (vide PL XIV. figs. 1, 2), and the brain-mass as a whole exhibits that 
separation and arrangement of its parts which permanently remain in Teleosteans. The 
mesencephalon embraces the largest extent of the brain, and by its prominence above 
imparts that rounded bulbous form to the head which is so characteristic of the young 
fish (PL VI. fig. 7; PL VIII. figs. 6, 7, 10; PL XIII. figs. 1, 5, 6 ; PL XIV. 
figs. 2, 4). Thus the medulla oblongata (mo), with the anterior transverse fold or 
cerebellum, forms a hind- brain plate of triangular shape, the mid-brain (mb) constitutes 
a similar triangular mass, and both have their broader sides or bases towards each other 
(PL VI. fig. 6), just as Kupffer describes in Clupea (No. 87, p. 220). The cerebellum is 
almost entirely covered by the posterior enlargements of the optic lobes (op, PL VI. 
fig. 7), but it protrudes distinctly as a thickened ridge passing across the front end of 


the fourth ventricle. A very thin roof continues from the cerebellum and covers the 
medulla oblongata, whose triangular floor is becoming better defined and dorso-ventrally 
deeper. Laterally the cerebellum (cb) does not break continuity with the double fold of 
the mesencephalon (mb), of which, indeed, it appears to be merely a thickened posterior 
portion or third fold (PL VI. figs. 5, 6). This fact has led many anatomists to deny to 
this fold the name of cerebellum. MM. Philipeaux and Vulpian regard it as merely a 
third lobe of the optic mass, and they, with many others who follow Weber (Anatornia 
comparata nervi sympathici, Leipzig, 1817), regard the two swellings passing along each 
side of the medulla oblongata, and composed of grey vesicular matter, as the cerebellum. 
MM. Philipeaux and Vulpian emphasise the view that the cerebellum consists of the 
two hollow lateral masses which flank that part of the optic mass usually called cerebellum 
(No. 118, p. 171). More recently Miklucho-Maclay has urged a similar view regarding 
the Elasmobranch brain, and he interprets the prominent anterior portions of the corpora 
restiformia as the true cerebellum. Beneath the cerebellum the medulla continues 
forward and merges in the basal region of the thalamencephalon, and even in this 
early condition distinctly turns upward, the curvature becoming marked somewhat later 
(PL XXIV. figs. 1, 2). Eyder states that in Alosa this upward bend is not indicated 
(No. 141, p. 504), but it is probably a notable feature in most Teleosteans. Dohrn, for 
instance, indicates it in Belone and the Lophobranchs, and the result of it is that the fore- 
brain is brought down below the front termination of the medulla oblongata, and a false 
cranial flexure is thus effected. The actual relations, in regard to position, of the brain- 
vesicles to, say the notochord, are not altered, nor does the head-region externally 
present a marked flexure downward (PL XIII. figs. 1, 5) ; yet a longitudinal section 
through the brain shows, as in the section of Anarrhichas (PL XXIV. fig. 2), the 
mesencephalon raised up, while the prosencephalic region bends down. If we follow 
the course of the medullary canal, we find in front of the cerebellum a marked 
ascent — the hollow optic lobes occupying a much higher plane than the thalamen- 
cephalon and the hemispheres, and the result is that, without the remarkable shifting 
forward of the parts of the brain seen in the chick, the prosencephalon comes to 
lie on the ventral side of the medulla and cerebellum. This modified flexure, which is 
not comparable to the true and extensive bending down of the prosencephalon, e.g., of 
Elasmobranchs, has this simple explanation, that without any very obvious displacement 
of the other parts of the brain, the floor of the myelencephalon and metencephalon are 
flexed upward, and, as a consequence, the mesencephalon is raised, and the thalamen- 
cephalon and hemispheres come to occupy a distinctly ventral position. This false 
flexure persists even after the embryo has emerged from the ovum, but with the fuller 
development of the oral region it is at a later post-larval stage corrected. 

The remarkable displacement just described does not change the external aspect of 
the embryo. The mid-brain still occupies the summit of the head — its greater pro- 
minence being due to the process of median upheaval from below, which brings the 
lateral margins of the optic lobes into proximity inferiorly with the eyes. The optic 


lobes, as before stated, laterally overlap the basal region of the mid-brain, at an early 
stage ; but this superposition is now considerably increased — indeed, it reaches fully to 
the middle lateral line behind the eyes (PL XXIV. fig. 3). The fore-brain still forms a 
narrow, laterally compressed mass projecting anteriorly to form the round snout or face 
of the embryo. Its bulk is less than half that of the mid-brain, and it encloses a small 
dorsal chamber (PL XXIV. fig. 5). The prosencephala floor is very dense, the side 
walls less so, while the roof thins out greatly. A small fissure continued from the 
chamber above passes into and partially divides the thick floor of the fore-brain; but 
sections of this region show a condition much in contrast with the capacious hollow 
vesicle of the Elasmobranch fore-brain. Several folds appear on the superficial aspect of 
the fore-brain (PL VIII. fig. 6) about the time of hatching or even before: but not until 
the second or third day after emerging does the deep fold appear which divides the fore- 
brain into two parts, the cerebral or anterior fore-brain and the thalamencephalic or 
posterior fore-brain (PL VI. fig. 7). A longitudinal fold passes over the dorsal surface of 
the fore-brain (fb) before it is markedly separated into front and hind prosencephalic 
regions, and it thus becomes longitudinally bifid at an early stage (PL VI. fig. 6). 
No olfactory lobes proper exist as yet; indeed, as Marshall found in Salmonoid larvae, 
these structures must be comparatively late in appearing. 

The changes which ensue when the primitive brain of three vesicles is finally divided 
into a series of five, are very complex and difficult to follow ; but the main features may 
be indicated. The five parts of the brain distinguished are as follows : — 

{(l) Anterior fore-brain or cerebral hemispheres, &c. (" Vorder- 
hirn," Gegenbaur and Baer). 
(2) Posterior fore-brain or thalami optici (" Zwischenhirn," Baer). 

n^r ii J (3) Mid-brain, or optic lobes, &c. (Mittelhirn, Baer, Zwischenhirn, 

I Gegenbaur). 

™ , 11 ( (4) Hind-brain or cerebellum (Hinterhirn, Baer, Mittelhirn, 

( Gegenbaur). 

Myelencephalon, (5) Medulla oblongata (Nachhirn, Gegenbaur). 

From the fourth ventricle a narrow fissure, the aqueductus Sylvii, leads into the third. 
The base of the latter partly overlies, and partly abuts against, the mass now separated, 
as already indicated, from the cerebral hemispheres or anterior fore-brain by a transverse 
superficial cleft (PL XIII. fig. 1). This posterior portion of the fore-brain is the 
thalamencephalon, and it is the part of the brain which, for the most part, overlies the 
roof of the mouth. 

Very early a portion of the hind part of the thalamencephalic floor is directed back- 
ward as a prolongation beneath the elevated medulla oblongata, and during its course its 
direction is slightly downward. The cells composing this encephalic diverticulum have a 
somewhat columnar arrangement, and surround a cavity continuous above with the third 


ventricle. The structure thus formed is the hollow infundibulum (inf, PI. XXIV. 
fig. 1), which abuts on the roof of the oral cavity below, though the two remain 
separate. The anterior part of this basal region gives origin to the optic nerves, w T hich 
will be considered under the sense-organs. A chamber, or rather a loose meshwork of 
cells (PL XXIV. fig. 1), most probably hypoblastic, though possibly mesoblastic, lies 
behind the infundibulum, and into this loose mass the oral end of the notochord (no) 
pushes as it bends downward. In some sections the notochord and infundibulum are 
brought into closer contact. The elevation of the oral roof too is very distinctly marked at 
this time, and such probably (see PI. XXIV. figs. 5, 6) corresponds to the curvature pro- 
duced in Elasmobranchs by the acute cranial flexure characteristic of those fishes and higher 
forms. On the summit of this arch a mass of cells appears, evidently a proliferation of 
the oral roof-cells rather than a diverticulum. This ovoid mass is the pituitary body 
(pt, PI. XXIV. fig. 1). It lies in front of the infundibulum, and from its origin is in 
close relation to the base of the thalamencephalon. The precise origin of this body in 
these forms is difficult to make out, but its cells, as Hoffman has clearly shown in the 
salmon and trout, are indistinguishable from the oral epithelium.* A small median 
swelling, not unlike the hypophysis in structure, lies in front of the latter — that is, behind 
and slightly under the point where the optic nerves decussate. When further advanced 
such appears to form the hypoaria or lobi inferiores — so well developed in Percoids, and 
their special ventricles in the adult communicate with the lumen of the infundibulum. 
The anterior fore-brain and the mid-brain at a very early stage so far overlap the 
intervening mass (the thalamencephalon) that only a small portion of its roof is super- 
ficially exposed (PI. XXIV. fig. l). This small extent of roof becomes very thin, as does 
also the roof of the anterior fore-brain, and it is much folded. In a transverse section 
through the mid-portion of the thalamencephalon before its walls have thinned out, a 
central aggregation of cells can be distinctly observed, and this soon exhibits a marked 
concentric arrangement, and become slowly pushed out as a papilliform process (PL VIII. 
fig. 6). A lumen develops at a later time, and it communicates with the (third) 
ventricle below. Its cells, which were rounded and not dissimilar to the adjacent cells of 
the thalamencephalic roof at this time, assume a columnar disposition, and it now forms 
that very prominent and interesting structure in young fishes — the pineal gland. The 
primary rounded or conical form is not long retained ; it either becomes truncated, i.e., 
depressed, or more or less plicated, and pressed against the thin developing arachnoid 
membrane, which alone separates it from the integument. In the salmon and trout 
Hoffman gives a slightly different account of its origin. It arises, he says, as a true 
evagination, not a solid protuberance, and its lumen is continuous with a portion of the 
ventricle below distinctly marked off as a special recessus infra-pinealis (No. 69, 
pp. 100, 102). Moreover, its cells are at first epithelial in character and columnar, 

* Dohbn states that the hypophysis makes its first appearance at the same time as the endodermal evaginations 
of the oral and branchial clefts. It arises as a pair of more or less distinct pouches much anterior to the paired oral 


whereas the rounded form is only assumed later when the cells have increased in number, 
and form two or three layers (No. 69, p. 103; cf. figs. 9, 12, Taf. iv.). Much later, when 
the absorption of the yolk is accomplished, the lumen of the epiphysis becomes obliterated, 
and it is separated off as an oval, lobed, or deeply folded solid mass of cells (No. 69, 
p. 103, Taf. iv. fig. 17). 

The spinal cord, when fairly advanced, proceeds quite to the termination of the 
notochord, but its general features call for no detailed description. Usually the terminal 
filum is very delicate and attenuated ; but at times a remarkable enlargement is observed. 
This final nervous swelling was very well seen in a young embryo of Motella mustela 
(PI. XV. fig. 4, ne), but in other forms it was also made out, e.g., Cottus scorpius 
(PL XIII. fig. 2) and Molva vulgaris (PL V. fig. 7). 

Auditory Organs. — The otocysts are very early differentiated — that is, about the 
same time that the lens of the eye is invaginated and defined (as pointed out by Kupffer, 
No. 88), i.e., in many pelagic forms about the fourth to the sixth day after fertilisa- 
tion. In Salmo solar, according to Parker, the ears are pushed in from the outside 
shortly before hatching; and he refers to these " auditory involutions " as "still widely 
open" during his "first stage" (No. 117, p. 113). This description, however, is quite 
unlike the mode of formation in the Teleosteans specially referred to in this paper; and 
Lereboullet's account, in the case of Esox lucius, is certainly more in accordance with 
observations at St Andrews, where he says that the early ears " are two small spheres, 
symmetrically placed, and formed by the grouping of plastic elements, .... at first 
solid; but becoming hollow, and transforming into the auditory capsules" (No. 93, 
p. 529). The otocysts are, in fact, not involutions of the external epiblast, but solid 
proliferations of the sensory or neurodermal epiblast (au, PL IV. figs. 4, 11, 16a). In 
Lepidosteus Balfour and Parker describe the ear as originating from the under or 
sensory layer, but as a hollow thickening, over which the epidermic layer is externally 
continuous (No. 18); and Hoffman, while he rightly speaks of this external layer 
as extending unbroken over the otocyst, says that the otocyst itself is formed as a 
hollow invagination of the under-layer (Grundschicht), a condition not exhibited by 
our sections of pelagic embryos. The earliest phase seems to be that of a rounded mass 
just becoming visible in the early haddock, i.e., a solid proliferation of the sensory 
stratum {conf. PL IV. fig. 11, the figure referred to, with Hoffman's, No. 69, figs. 3 
and 4, Taf. i., and fig. 1, Taf. iv.), in which very soon a radial arrangement of cells can 
be made out preparatory to the formation of a lumen. The latter rapidly appears 
{au, PL IV. fig. 13 ; PL V. fig. 8), and is at first minute and spherical, but soon 
enlarges to form a spacious ellipsoidal chamber [au, PL VI. figs. 5, 6), very obtusely 
rounded, depressed laterally, and with its inner wall abutting against the neurochord (ne), 
while on its outer side, and superiorly, it is separated from the exterior only by the 
tegumentary epiblast (ep). The walls of the otocyst are very dense when the lumen is 
.small (au, PL IV. fig. 13), but they apparently stretch as the chamber expands, and 
become comparatively thin (au. PL V. fig. 9; PL VI. figs. 1, 2, 7). Lereboullet 


noticed that, as the auditory vesicles elongate, "a mass of yellowish granules" appeared 
in them prior to the formation of the true otoliths (No. 93, p. 529). The contents of the 
otocysts seem, however, to be clear, homogeneous, and without granules in our forms, but 
usually before the end of the first week, and within twenty-four hours after the lumen 
in each is defined, two minute calcareous bodies appear on the floor, usually towards 
each extremity of the longer axis of the otocyst (oto, PI. VI. fig. 5; PI. XII. figs. 1-5). 
These otoliths have the appearance of two very small dense grains, and are, as Dr 
Carpenter remarks (No. 37), similar in character and mode of formation to the concre- 
tionary spheroids common in the urine of the horse, the integument of the shrimp, and 
other forms. It is well known that when a solution of lime-salt in gum-arabic is slowly 
decomposed, carbonate of lime is deposited in spheroidal concretions. Sometimes, as Mr 
Rainey found, two of these will unite in dumb-bell form, and occasionally a number will 
unite in the form of a mulberry (No. 126, p. 19). # The walls of each otocyst are com- 
posed of columnar or rather spindle-shaped cells, and at first over much of their surface 
several layers are superposed (PI. VI. figs. 3, 4). Subsequent changes, however, not 
only affect the thickness of the walls, and cause them to thin out, but alter their contour. 
Moreover, being pressed in, from above, anteriorly, the otocyst (au) when viewed from 
the side, loses its angular elliptical shape, and has more or less the outline of an oyster- 
shell (PI. VIII. figs. 4, 6, 8, 9; also PI. XII. figs. 1-4, 7). A ridge also appears on the 
floor, caused apparently by some of the internal nervous tissue being aggregated along 
the shorter axis of the capsule (PI. VIII. fig. 8). In Esox, about the time the cardiac 
chamber is formed, and the embryo rises erect upon the yolk, the otocysts, according 
to Lereboullet, become more transparent, and have a thick investment like cartilage 
(No. 93, p. 529). No such investment appears in our forms until very much later, the 
walls retaining their original cellular structure (PI. VI. figs. 3, 4), though at certain 
points they become thickened, sensory cushions (neu) being formed of large fusiform 
cells, which take a 'slightly radial disposition. An embryo of T. gurnardus, six days 
after hatching, shows three such nervous aggregations provided with erect motionless 
cilia or palpocils. That situated upon the floor is by far the largest, but it may vary 
somewhat in outline as well as position. The remaining two are anterior and posterior 
(PI. VI. fig. 2). In one specimen, viz., the example figured, a dorsal hernia (x) or 
process of the cellular wall occurred. A long trumpet-shaped tunnel (can) passed 
anteriorly and superiorly, the inner end being faintly granular and botryoidal in appear- 
ance from the irregularity of its cells. In some forms the ears become so enormously 
developed that they may nearly meet in the middle dorsal line, or may, as Parker 
describes in Salmo, actually overlap the posterior border of the eyes (No. 117, p. 113). 
In the gunnel (au, PL XIII. figs. 5-7) they are certainly very much larger at a com- 
paratively early stage than in any other form reared at St Andrews, and they may 

* A still more striking example of definite concretions in a clear fluid is that afforded by certain Annelids, e.g., 
the stylets of the Nemerteans. Lereboullet proved their calcareous nature in fishes ; he says — "Treated with acid 
they effervesce and disappear. No membrane is left, or it is too thin to distinguish " (No. 93, p. 633). 

VOL. XXXV. PART III. (NO. 19). 6 E 


be larger and more rapidly developed in the embryos of demersal ova than in pelagic 
forms, though in the clupeoids with both demersal and pelagic, ova the spacious nature 
of the otocysts is a characteristic feature. 

It is difficult to follow the changes in the structure of the ear in the transparent 
living fish, on account of its complexity. The semicircular canals seem to be developed 
primarily from thickenings of the cellular lining of the auditory sac, like the nervous 
cushions, a soft hernia being produced which grows inward as an increasing ridge, in 
which a cavity is formed, as shown in PL VI. fig. 10, can. Viewed from the side, at 
a later stage the semicircular canals protrude into the chamber of the otocyst as three 
short knobbed processes directed inward from the margin. The median and inferior 
canals end abruptly in the middle of the chamber. Frequently the otoliths, instead of 
lying apart, each in a depression of the auditory floor, may shift, so as to lie towards the 
same part of the otocyst, e.g., in the anterior depression, as in PL VI. fig. 7, and PL XII. 
fig. 7. At times three otoliths occur, and when two are present, as is normally the 
case, there is usually a marked disparity in size, Lereboullet remarking that in 
Perca jluviatilis the posterior otolith acquires a diameter triple that of the anterior 
(No. 93, p. 632). 

In preparations very deeply stained with hsematoxylin the otoliths not only show the 
usual glistening crystalline structure with radial striations (oto, PL VI. figs. 2, 3, 4, 9), 
but less numerous concentric striations, and a very marked dark central core surrounded 
by an external stratum, which stains more faintly (PL VI. fig. 11). 

A further stage in the development of the Teleostean ear is observed in the young 
flounder (PL XV. fig. 8), in which the disparity of the otoliths and the complex 
condition of the auditory chamber are well shown. 

Olfactory Nerves and Pits.— The olfactory pits are distinguishable on the sixth day 
or later, i.e., about the time that the heart's pulsations commence. They are produced 
by a paired thickening of the sensory epiblast (ep 2 , PL IV. fig. 17) in front of the upper 
part of the hemispheres. Each soon forms a flattened oval sac of slightly elongated cells 
(ol, PL IV. fig. 2), beyond which a small portion of the fore-brain (fb) extends (PL IV. 
fig. l). A depression commences from the outside, and each nasal sac becomes a cup- 
like structure, whose cells are now fusiform and radially arranged (ol, PL IV. fig. 17). 
The flattened corneous layer is no longer present at the two points where the pits are 
formed, and as they become deeper and the walls of each sac increase in thickness, they 
may be brought into close contact with the anterior fore-brain, upon whose front they 
seem to lie in the living embryo (PL VI. figs. 6, 7, 8, 10 ; see also PL XII. figs. 1, 3, 
7, and PL XIII. figs. 1, 3, 5, 6, 7). So small is the space at this time separating 
the sacs from the brain that it is difficult to detect the nerve-strand which connects 
them. Hoffman, however, made out the origin of the olfactory nerves as minute pro- 
liferations of the wall of the anterior fore-brain (No. 69, p. 87). This minute outgrowth, 
on reaching the nasal sac, coalesces with the proximal surface of the nasal pit. No olfactory 
lobes are at this time discernible ; indeed, Marshall doubts whether in the Teleosteans 


(Salmonoids) he examined true olfactory lobes ever are formed. At any rate, he cannot 
regard them, and justifiably so, as embryonic structures (No. 100, p. 313). The proximity 
of the olfactory pits and the brain renders the determination of such a point in the 
minute Teleostean forms here considered very difficult ; but Marshall's conclusions 
admit of little question. In the Elasmobranch- embryo the olfactory lobes are not 
distinguished until almost all the features of the adult are attained (Balfour's stage 0) 
(No. 100, vide pi. xiv. figs. 24, 33, 34), and in the chick they cannot be made out 
until the seventh day (No. 17, p. 162). There is no trace of these lobes in Rana during 
the earlier stages, according to Marshall, and the nerve-strand passing to the olfactory 
pit is very short. 

A similar condition is found in Teleosteans ; a solid strand of cells passes from the 
roof of the fore-brain, before it shows any trace of external division, to the pits (ol, 
PI. IV. fig. 16), and these latter as they increase in bulk approach, as in PI. IV. 
fig. 17, and come into such close proximity to the fore-brain (fb) that an actual 
reduction in length of the primitive nerve results, so that it is barely distinguishable 
(PI. VI. fig. 6). The histological character of these primary olfactory strands supports 
the view that they are merely diverticula from the brain, in which organ no fibres are 
yet formed, for the first pair of nerves have a similar solid cellular structure. This 
structure Marshall found to be retained, when the other cranial nerves had assumed 
the fibrillar character. It is remarkable that the olfactory nerves, which are amongst 
the earliest to be given off, retain their primitive structure longest. Marshall could 
not make out any ganglionic enlargement (No. 100, p. 312); but Beard in some later 
researches found that, as in Rana, a ganglion does arise in connection with the epiblastic 
thickening forming the pit, and that the olfactory nerve itself is also split off from 
the skin {vide his figs, of Rana and Rhodeus amarus, figs. 3 and 4, pi. viii. No. 22). 
The dorsal position of the nasal pits is interesting, as in the Elasmobranchii and Aves 
these structures are on the under side of the head. The nerves shift down from their 
first position, and are found to connect with the fore-brain ventrally (1, PI. XXIV. 
fig. 4; also vide Marshall, No. 100, pi. xiv. fig. 33). Of course in the Teleosteans this 
transference is much reduced, as the fore-brain does not grow so extensively as the hinder 
portions of the brain ; but Marshall has undoubtedly given accurately the facts of 
the early development of the first pair of nerves, which, however, Huxley considered 
to be developed late, and to have but one paired connection with the brain, and that a 
ventral connection (No. 74, p. 71). This ventral origin is secondary, and comparatively 
late, but it is very much later before the basal swellings known as the olfactory lobes 
are clearly indicated (PI. XXIV. fig. 4). 

In the early forms treated of in this section the division of the original single nasal 
opening into two was not followed. It is readily observed in the wolf-fish (Anarrhichas) * 
and in the young flounder (PI. XV. fig. 8). 

Optic Nerves and Vesicles. — One of the earliest features in the development of the 

* Vide section xiii. p. 254. 


Teleostean embryo, as already noted, is the enormous development of the anterior 
cephalic region, which is chiefly due to the protrusion of two rounded lateral masses from 
the sides of the narrow fore-brain (PI. V. fig. 1, op), and not, as Lereboullet stated, 
from the walls of the mid-brain (No. 93, p. 522). The pair of massive bulbs thus 
formed are rapidly defined as the ellipsoidal optic vesicles, the first of the sensory 
organs to appear. In section (PI. IV. fig. 1 6) the cells of the neurochord, at a point 
midway between the dorsal and the ventral surface, actively push their way outward, 
and pass for the most part upward, so that a pair of stalked vesicles are formed, lying 
against the sides of the fore-brain — not quite upright, but placed at an angle which 
brings the lower and smaller lobe in proximity, while the upper and much larger lobe is 
pushed away from the brain (PI. IV. fig. 3). Sections clearly demonstrate the abundant 
protrusion of cells to form the optic bulbs, which Kingsley and Conn regard as formed 
in the main by a constriction or fissure commencing above and behind the lateral 
enlargement, and progressing forward and downward (No. 78, p. 207), but the constriction 
which they carefully describe is preceded by a very apparent bulbous outgrowth. These 
protruded cells are indistinguishable in size or contour from the neurochordal cells which 
gave them origin, but the outer limiting layer of cells assumes a columnar disposition, as 
also does a double plate of cells along the median dorso-ventral plane. This latter feature 
has been referred to (No. 122, p. 452) as a radial disposition of the central cells and 
"as though about to dehisce along a central vertical plane in order to form a median 
chamber, longitudinally placed ; " but a chamber converting the solid optic proliferations 
into capacious hollow vesicles, such as the early condition of these structures is generally 
described, is never completed — a very narrow fissure being all that is usually formed, and 
even this may at times fail to be developed before the invagination of epiblast presses 
the outer layer against the inner layer of the primitive optic vesicle. Ryder describes 
and figures the narrow fissure referred to (No. 141, p. 499, pi. v. figs. 26, 27); and 
in section (PI. IV. fig. 17) it is plainly seen as a slit in the midst of the optic vesicle. 
This separation of the median cells is interesting, for, though the Teleostean eye is not 
pushed out as a chambered sac from a hollow brain-vesicle, as is the primitive mode of 
origin, it secondarily acquires a trace of this vesicular condition. In the living embryo 
it rarely presents more than the character of a delicate median line or slit in the optic 
bulb (PI. V. fig. l). PI. IV. fig. 3, shows the first indication of this slit-like lumen, 
which can be traced along the short thick stalk into the fore-brain, where it is lost. In 
horizontal section we see that while the cells — pushed out to form the optic vesicle — in the 
main pass upward, they also extend posteriorly, carrying the vesicle some distance behind 
its pedicel or point of origin: the optic vesicles on their appearance are thus defined 
most distinctly behind. Each vesicle, in fact, forms a depressed pyriform body, 
which by its smaller end remains attached to the brain, while the swollen upper 
portion extends dorsally, backward and distally (PI. IV. fig. 16). Schenk, who first 
gave a full account in his well-known researches on the eye of fishes (No. 143), seems 
not to have recognised the fact that the eye and the entire central nervous system in 


these forms is primarily solid and without a lumen. Kupffee appears to have been the 
first to describe the true condition (No. 88). 

It is important to notice that the nerves — that is, the stalks of the optic vesicles — arise 
at a different level from the olfactory and other nerves, a fact inconsistent with the 
derivation of the nerves from a ridge or " sinnesplate," such as Gotte and others 
distinguish. A dorsal stratum of neurochordal cells may perhaps be regarded as a 
neural crest from which the posterior nerves spring, but such a crest does not pass 
further forward than the hind-brain ; the first and second pair of cranial nerves, as will be 
seen, arise, the one primarily as dorsal and the other as lateral median evaginations of the 
prosencephalon. In pushing backward the optic vesicles shut off a thin stratum probably 
of mesoblast which later forms an enveloping cup, and gives origin to some important 
structures in the developing eye. This mesoblast (mes, PL IV. fig. 17) is probably a 
forward growth from the thin plate of the same layer in the otocystic region (PI. IV 7 . 
figs. 16, 17). Meanwhile, the short, thick connecting stalk becomes constricted, and 
the vesicle itself alters both in form and position. Viewed from the side, the latter 
is now almost perfectly elliptical (op., PL XXII. fig. 12), and is nearly perpendicular in 
position, i.e., parallel to the vertical axial plane of the embryo (PL V. figs. 3, 10). 
The columnar cells along the central vertical plane of each vesicle (PL IV. fig. 3) 
separate sufficiently to mark a slight but distinct fissure (PL IV. fig. 17; PI. V. 
fig. 1). This fissure persists when the optic vesicles have altered their position, so that they 
lean by their upper portion against the neurochord, and this median chamber, instead of 
passing upward, outward, and posteriorly, as when first indicated (PL V. fig. ]), now 
passes downward and outward (op, PI. IV. fig. 14). As already indicated, the pyriform 
outline is almost wholly lost, the optic vesicles lying obliquely against the fore and mid- 
brain — as elliptical bodies laterally flattened, and traversed by a vertical lumen longi- 
tudinally separating each into an inner and an outer half, the latter layer being very 
much thicker than the inner half (PL IV. fig. 17). This condition does not remain long. 
Before the end of the day these " primitive optic vesicles " become indented by the pressure 
of the epiblast lying external to them, the deeper layer of which becomes rapidly thickened 
so as to form in section (PL IV. fig. 17) an almond-shaped mass on each side, pressing upon 
the central region of each optic vesicle (op), which gradually becomes cup-shaped, the hollow 
of the cup being occupied by the thickened mass of epiblast, which forms a dense spherical 
body, the lens (I, PL IV. fig. 14). The optic cup or secondary optic vesicle becomes thinner 
marginally, and this portion creeps round to the outer side of the lens (PL IV. fig. 21), 
forming a circular lip around it, which is incomplete on the lower side. This gap, the 
choroidal fissure, is very distinctly seen at this stage (ch, PL VIII. figs. 6, 7, 8; PL 
IX. figs. 1, 3; PL XII. figs. 1, 2), and it persists for some time (PI. XVI. fig. l). 

The mesoblastic cells, which were included as a thin plate between the optic 
vesicle and the brain (mes, PL IV. fig. 17), have spread over the former as an outer 
layer (PI. IV. fig. 21), and pushed their way through the choroidal fissure into the 
interior chamber of the eye, as is seen in section (PL IV. fig. 20). A similar horizontal 


section, at a lower plane (PI. IV. fig. 19), shows the fissure disappearing. These intruding 
mesoblastic cells (mes) appear to become packed between the lens and the retinal surface 
of the optic cup, and doubtless break down to constitute the vitreous humour of the adult 
eye, forming also, as some observers think, the " capsula hyaloidea," in which a rich 
vascular network afterwards develops. The differentiation of the cellular optic vesicle 
into its various layers has already taken place before the embryo has emerged from the 
ovum. The formation of these layers can, however, only be very briefly touched upon.* 

We have seen that the eye, soon after its appearance as a solid bulbous protrusion 
(op, PI. IV. fig. 16), separates by a slight fissure into two layers, constituting the primary 
vesicle (PI. IV. fig. 17). With the obliteration of the lumen, the two layers become 
closely apposed, and the eye consists of a thick-walled cup of undifferentiated cells 
(PI. IV. figs. 14, 21), whose chamber — the lumen of the secondary vesicle — is closed 
in front by the growing lens (I). An investing layer of mesoblast (mes) forms the 
sclerotico-choroidal sheath, absent, however, from the front of the eye. As the time for 
extrusion approaches, scattered pigment-spots occur outside the optic vesicle and in the 
external investment. These spots are unbranched amorphous particles, sparsely 
distributed as an irregular pigment-layer over the whole surface of the optic cup, save 
in front of the aperture of the pupil (PI. XIV. fig. 1; PI. XVII. fig. 10). On each 
side of the lens they are densely aggregated (PI. V. fig. 6 ; PL XVI. fig. 8). The 
outermost layer of the cellular vesicle, i.e., the stratum of cells internal to the layer of 
pigment, assumes a marked columnar character (PL XL figs. 6-8) — bold striae passing 
across it, and dividing it into wedge-shaped radiate masses as indicated in PL XXIII. fig. 3«. 
At the same time the cells within, constituting the main bulk of the vesicle, separate, 
though somewhat obscurely, into two layers of great and almost equal thickness — the inner 
layer being slightly thicker (in section) than the outer. The line of separation is delicate 
and indistinct at first, but subsequently develops into a fine molecular band — the prominent 
internal molecular layer. The inner surface of the columnar stratum shows a delicate 
membrane, possibly the posterior or membrana limitans externa (" limitans interna " of 
Hoffman, No. 69, p. 50).t From the outer stratum consisting of columnar elements 
the rods and cones are developed, while the two thick inner layers with their intervening 
lamina give rise to the other layers of the retina. Such is the condition of the six 
layers of the retina shortly before the time of hatching in pelagic forms, e.g., the cod and the 
haddock. In other forms, chiefly demersal, which reach a somewhat advanced embryonic 
stage while within the ovum, the eye attains a much further degree of development. 

A haddock, on the second or third day after extrusion, shows additional changes, the 
second layer being better marked, as is also the inner molecular layer, though both 
are still very thin laminae. The columnar character of the bacillar stratum is still more 

* The minute description of the development of the Teleostean eye is in the able hands of Dr Marcus Gunn, one 
of the surgeons at Moorfields. 

t This layer Dr Gunn has identified as the "external molecular layer" — a "thin dark finely granular line;" 
and should this he so, then the " limitcms externa" must he developed at a much later stage, as Dr Gunn states. — Vide 
Ann, Nut. Hist., Sept. 1888, p. 268. 


distinct. During the first week after hatching, the internal molecular layer undergoes 
great development, and rapidly becomes a thick and bold stratum separating the external 
granular layer from the internal granular layer. The external molecular stratum is 
slowly differentiated on the inner surface of the columnar layer, while the inner 
granular layer shows, though very obscurely, a separation into an inner and an outer 
portion. The pigment of the choroid is much more abundant than before, and in the 
living embryo gives to the eyes a dense appearance, so that the minute transparent 
fish can usually be discerned in the tanks of the laboratory by the two large dark eyes, 
which form a most prominent feature (PI. XVI. figs. 3, 6, 7* 9 ; PI. XVIII. figs. 1, 2). The 
structure of the retina exhibits little further change during the later larval stages, but in 
the post-larval conditions other features appear, which need not be noticed in detail by 
us, as Dr Marcus Gunn has specially occupied himself with this subject. Thus in a 
young flounder, still transparent and colourless, the pigment-layer is greatly increased in 
thickness, and it sends prolongations into the bacillary layer. The cylindrical rods 
form a very distinct stratum, while the flask-shaped cones are well defined, and present 
a contrast to the corresponding layer in Amphibians, which have a very insignificant 
stratum of cones. Indeed, as Max Schultze pointed out, this layer in Teleosteans recalls 
the condition in the Mammalian retina (No. 144, vide sect. iv. of his paper). The 
double disposition (twin-cones) in the adult eye of osseous fishes has not yet been 
assumed, so far as can be made out. The striking coloured globules so prominently seen 
in this layer in Batrachians, birds, and some reptiles are absent, nor do they at any 
subsequent stage appear to be developed. That Teleosteans should have a layer of rods 
and cones so early and so well developed, whereas in Selachians (and cartilaginous fishes 
generally, it is said) no cones can be made out, is a remarkable circumstance. Bats, 
hedgehogs, and other nocturnal forms amongst Mammals, are destitute of cones. 

The limitans externa in the post-larval stages is a very delicate lamina ; but it is 
well defined. The external granular stratum now consists of several layers of large 
cells separated from the inner granular layer by a comparatively broad external mole- 
cular layer. 

The inner granular layer itself Hoffman separates into three portions — an outer thin 
stratum of "tangentiale Fulcrumzellen," a " medialer Theil der inneren Kornerschicht," 
and a " lateraler Theil " of the same layer. In the flounder, as well as in such forms as 
Cottus and Cyclopterus, only the outer "tangential " cells can be distinguished from the 
remaining elements of the inner granular layer, which form a very thick band. Internal 
to the last-named layer is the internal molecular stratum, anterior to which the 
ganglionic layer can be distinguished. The internal molecular layer Hulke describes as 
including a large quantity of connective tissue, in the midst of the fibres of which are 
large branched corpuscles of very considerable dimensions (No. 71, p. 247), but in com- 
paratively late post-larval stages no trace of these structures can be made out. The 

* Mr Cunningham's figure (op. cit., pi. vi. fig. 4) appears to be, as he supposes, this species, viz., Liparis 


ganglionic layer is composed of large cells, which form a remarkably broad layer — quite 
unlike the narrow ganglionic stratum in the Salmonidse. Anteriorly it is defined by the 
fibres of the optic nerve, and the limitans interna (Hoffman's "limitans externa") or 
anterior limiting membrane, which forms the lining of the optic globe. Some observers 
look upon this membrane as the hyaloid capsule of the vitreous humour ; Hulke, 
however, regards it as a separate membrane, and such it would appear to be, since it 
precedes the formation of the vitreous fluid by a long interval (vide No. 71, p. 248). 
An anterior annulus, the lip of the secondary optic vesicle or cup, remains unaffected 
by these histological changes, and a mass of indifferent cells fills up the interspace between 
the retina proper and the circular curtain — the extension of the choroid in front of the 
eye. These cells are in fact involved in the formation of the iris and ciliary ridges, the 
ciliary muscles being developed from the mesoblast (mes) entering by the choroidal fissure 
(PI. IV. figs. 19, 20). Even in the later larval stages this complex anterior annulus, 
formed of the cells just mentioned, and the pigmented choroid which grows round to 
enclose a circular opening in front of the eye — the pupil, constitutes a brilliantly 
opalescent iris, which adds to the remarkable appearance of the minute transparent 
larva (PI. XVI. figs. 3, 7, 9 ; PL XVIII. fig. 11). 

Cranial Nerves. — The optic, olfactory, and auditory nerves are treated elsewhere, and 
in this place only the larger and more important nerve-origins will be referred to, the 
Teleostean embryo being little favourable for tracing the development of the smaller 
cranial nerves, such as III., IV., and VI. The trigeminal (V.) is large, and readily made 
out. In Elasmobranchs it arises as two lateral outgrowths from a median dorsal ridge at 
the anterior end of the hind-brain. At a late embryonic stage this nerve springs from 
the upper lateral margin of the hind-brain, but so far forward that the optic lobe covers 
it at this point, and it appears to emerge from the overlapping lobes at a point imme- 
diately posterior to the eyes. This lateral position must be secondary (as Marshall 
suggests in the case of Scyllium), the original median dorsal position being altered by the 
rapid growth of the roof of the brain, so that the origins of this pair of nerves become 
further and further separated, until finally they are lateral (No. 101). Just as the 
nerve emerges it separates into several rami, the outermost being the maxillo-palatine 
branch, while a second large branch, the mandibular, passes backward a short distance in 
close contact with the side of the medulla oblongata. Each of these main rami shows, near 
its origin, a very large ganglion, the two ganglia being so close together as to appear 
like slightly separated moieties of one primary ganglionic swelling. From the ganglion of 
the maxillary nerve a small nervous branch passes forward over the orbital arch, possibly 
the abducens (VI.), though more probably it is the ramus ophthalmicus of the Vlth nerve. 
Between the two main rami just mentioned a large blood-vessel passes, and a third 
ganglion appears beneath it, also apparently one of the trigeminal group, while a slender 
nerve, whose destination could not be made out, was connected with this smaller ganglion. 
A little posterior to the trigeminal the Vllth and Vlllth arise in close proximity to each 
other, the auditory being posterior and exhibiting a large ganglion. 


It is difficult to follow the fibres of two nerves, so contiguous, to their centres in the 
brain ; but fibres can be traced from the upper lateral edge of the medulla over 
a wide curve which brings them near the base of the third ventricle, or more correctly 
above the pyramids ; these must belong to the facialis ; and the auditory (VIII.) consists of 
those fibres which come out close to the surface of the medulla just below the overlapping 
posterior part of the optic lobe. These two nerves, in regard to their nuclei, thus are 
widely separated ; but where they arise from their common site on the upper margin of 
the medulla they are separable only by the fact that the fibres of the facialis pass down to 
the mandible and posterior margin of the hymandibular cartilage ; while the VHIth nerve 
has a very short course, breaking up on the under surface of the auditory sac to supply 
at least three special sensory areas (ncu, PL VI. figs. 3, 4, 9, 10) in the otocystic chamber, 
and forming a prominent ganglion outside the ear before doing so. Of the glosso- 
pharyngeal nothing can be said here, but the vagus (X.) apparently arises by two complex 
roots ; the first, which probably includes the fibres of the IXth nerve, issuing from a point 
near the lateral summit of the medulla oblongata, which point is in the same transverse 
plane as the oral termination of the notochord. It passes along the side of the medulla 
and penetrates the auditory cartilage, sending twigs apparently to the four gill-arches and 
to the pharynx. The nucleus of this portion of the vagus is confined to the superficial 
swelling of the lateral ridge of the medulla. Not so with the second part of the vagus. 
Its fibres describe an arch or curve, and can be traced to the median region of the medulla 
below the floor of the fourth ventricle and above the pyramids, while part of its fibres have 
a more superficial origin. On emerging they form a very massive, prominent root, passing 
in the main through the hind part of the ear-capsule, just above the thick basilar plate 
where it is in contact with the otocyst, and forming in front of the pectoral girdle a large 
double ganglion below and to some extent internal to the ear. The section which shows 
this bifid ganglionic mass presents another ganglion, apparently the ganglion of the first 
part of the complex. This ganglion is smaller, somewhat higher, and posterior to the 
large double ganglion. The former lies on the inner side of the anterior cardinal trunk, 
below which is a slender ganglion, whence twigs can be traced to the opercular region and 
to the skin, forming, between the muscle-plates and the neurodermis, a nervous tract, 
probably the origin of the lateral line. 

The large double ganglion first named lies just above and external to the pronephric 
swelling, the intimate relation of the two structures being noteworthy. Its fibres go, 
as before said, to the pharynx and the branchial arches. From the smaller ganglion, 
described above, pharyngeal and important cardiac branches also pass. 

Lateral Sense-Organs. — Little can be added to the observations of Hoffman (No. 69) 
with reference to the development of the lateral sense-organs. In a young gurnard, 
about eight days old, they are very distinctly seen in the transparent though somewhat 
corrugated and glandular integument. Generally three or four can be made out in the 
haddock, one on the top of the head, just behind the eyes, a second situated a short distance 
behind the pectoral fin (see PL XVII. fig. 1), while one or two occur along the caudal trunk. 

VOL. XXXV. PART III. (NO. 19). 6 F 


They are not, however, regularly arranged, and the distal enlargement protruding from 
the integument is often absent. Thus, in the gurnard above referred to, nerve-filaments 
were observed passing across the sub-epidermal space from the trunk, and terminating in 
the skin without an enlarged sensory-organ. The external sensory-organ (PL VI. figs. 
8, 8a) consists of a somewhat elliptical aggregation of granular columnar cells, from which 
a number of very fine and apparently rigidly erect palpocils (pip) project. A delicate 
nerve-filament (nv) passes from it to the muscular plates (my), and so to the central 
nervous system. This filament shows a slight enlargement at its proximal end, and 
another dilatation just as it approaches the external sensory-organ. 

As noted on a previous page, large spaces (ss) filled with a clear plasma exist below 
the integument (ep), separating it widely from the trunk (PL XV. fig. 7), and across 
these spaces in more advanced embryos fine nervous threads pass from the myotomes to 
the skin, occasionally giving off in their course delicate secondary filaments. The nerve 
going to the cephalic sensory-organ apparently comes from a cutaneous sensory branch ; 
and Hoffman states (No. 69) that the development of the ramus lateralis nervi vagi 
always precedes the appearance of these sensory-organs. 

No sections of the early stages show the longitudinal sensory tract called the " lateral 
line " in fishes. There is, however, in the caudal trunk of an advanced haddock a canal 
apparently surrounded by nervous cells and mucous tissue which stains deeply (PL XL 
fig. 16), but it can only be traced a short distance in the tail of the example referred to. 
As noticed elsewhere, the facial region is provided with numerous papilliform sensory 
bodies, and these are large and very noticeable in what may be called the maxillary or 
sub-prosencephalic region. They exhibit a structure similar to the lateral sensory-organs, 
and are composed of lengthened spindle-shaped cells (PL XXI. fig. 7, sb). 

Alimentary Canal. — In its earliest condition the alimentary tract consists merely of 
a thickened sub-embryonic layer of hypoblast, intervening between the neurochord above 
and the yolk, or rather periblastic cortex of the yolk, below. Posteriorly, when little 
more than one-third of the yolk is covered by the blastoderm, the hypoblastic cells 
beneath the embryonic axis, as already pointed out, assume a distinct columnar character 
(hy) ; a lumen (hg) appears below, which is arched over by columnar hypoblast, and 
has a floor of nucleated periblast (per, PL IV. figs. 56, 6). This is the first indication of 
the alimentary tube, and it forms the posterior section — the continuity of which with the 
neurenteric canal and medullary groove has been already described. From the arched 
enteric roof the notochord is differentiated. The lumen at first extends but a very 
short distance forward, and is lost in an anterior aggregation of hypoblastic cells. These 
cells, formed by the proliferation of the thin sheet of invaginated hypoblast, reach as 
far as the cardiac region, where they thin out rapidly, and form a delicate limiting 
membrane below the head (hy, PL IV. figs. 3, 4). As this thickened mesenteric mass 
arises, the embryo is necessarily raised from the yolk except in the cephalic region — 
the snout still lying in close contact with the yolk below, so that a pseudo-cranial 
flexure is produced, and a pericardial space (pd) formed beneath the otocystic region 


At the sides of this space, i.e., beneath the eyes, the hypoblast becomes thickened 
as two lateral longitudinal ridges (PL IX. fig. 1 ; PL XL fig. 1), but elsewhere in this 
region the layer forms a very thin plate (PL IV. fig. 21). That the roof of the primi- 
tive enteron is thus formed as a dorsal sheet of invaginated hypoblast, admits of no 
doubt. Such sections as figs. 56, 6, and 10, PL IV., demonstrate this, and the ventral 
wall of the canal is formed of cells either pushed in from the side — that is, formed of true 
hypoblastic cells or aggregated as masses of protoplasm around the scattered nuclei of the 
periblast, and budded off. While the posterior portion is formed in this way, the mesen- 
teron proper appears to develop in a different manner, being formed by a multiplication 
of the invaginated (hypoblastic) cells ; and a ventral and a dorsal wall are not definitely 
formed from periblastic and hypoblastic cells respectively, but doubtless periblastic cells 
contribute in some degree to build up this portion of the tract also, though in such 
sections as figs. 13 and 14, PL IV., the hypoblast (hy) is a very definite and continuous 
layer. The mid and fore portions form a dense cord, in which a lumen appears later by 
the forward extension of the posterior enteric chamber, this oesophageal slit extending in the 
ling, two days old, in front of the otocystic region. At first the hind gut is open to the 
yolk below (as in PL IV. figs. 5b, 6), but no sections show this to be true of the enteric 
tract further forward. According to Hoffman, paired involutions of hypoblast produce 
the tract which he thus holds to be open to the yolk beneath (vide No. 69a, Taf. i. fig. 3), 
but no section in pelagic forms indicates such a mode of origin of the mesenteron, and 
certainly not of its oesophageal portion. The earliest condition of the alimentary tract is 
a continuous sheet of hypoblast, thickened on each side in the oral region to form the 
lateral walls of the oral chamber. Lereboullet regards the alimentary canal as developed 
by a folding-in of the " mucous layer," though the pharyngeal section, he holds, is not 
formed till later. In his earlier researches he states that the enteric tract is possibly 
formed by " une vegetation celluleuse," such as Vogt had described as involved in the 
formation, not only of the intestinal tube, but of the liver and kidneys (No. 93, p. 538). 
Dohrn believes that the oral hypoblast is a forward growth of the mesenteric mass, nor 
is there evidence to show that this is not so. At any rate, in the embryo whose optic 
vesicles are in process of formation, the hypoblast (hy, PL IV. figs. 4, 13, 14, 20) is a thin 
sheet — a single layer of cells for the most part over the entire ventral surface, save at the 
posterior extremity (hy, PL IV. fig. 10). At a somewhat later date, when the invagina- 
tion of the lenses is completed, the mesenteron is a massive cylinder, and the oral tract a 
wide flattened sheet of hypoblast formed either by proliferation of the invaginated layer, 
or by forward growth of the denser hind gut, probably a combination of both. In any 
case, Lerebotjllet's view is the correct one, viz., that the pharynx is a separate and later 
formation than the mesenteron proper. By the time the walls of the otocysts have 
thinned out and their chamber has enlarged and contains the otoliths, a fine horizontal 
fissure traverses the pharynx, and a lumen thus continues from the oral to the blind anal 
end of the alimentary canal (PL IV. fig. 11). The cells now assume the full cubical 
columnar character characteristic of the enteric epithelium, and, at first a single layer 


(PI. IV. figs. 11, 19), they increase until the enteric walls are thick, and include many 
liters of wedge-shaped cells (PL VII. figs. 7, 9). In P. fiesus of the ninth day {i.e., two 
days before hatching) the walls are just "00 1 inch in thickness, and the lumen in the 
middle or widest part measures in horizontal breadth slightly less. A delicate inner layer 
lines the lumen, which has a granular or mucoid appearance, but it subsequently forms a 
ciliated enteric lining. It is not more than '000125 inch in thickness. The lumen of 
the mid gut is large and round in transverse section (mg, PI. VII. fig. 3), but much more 
depressed further forward. A section through the pectoral region, where the enteron is 
oval and the lumen a wide transverse fissure, shows a diminished dorso-ventral capacity, 
while in the oral region proper a mere horizontal slit extends from side to side of the 
wide and very much depressed layer of oesophageal hypoblast (PI. XL figs. 2-8). The 
tract is thus a closed sac (PL IV. fig. 12), flattened anteriorly, round and cylindrical poste- 
riorly, the mouth and anus being " the last parts," as Lereboullet said, "to be formed." 
Around this tube of hypoblastic cells the splanchnopleure (sp) grows, forming a thin 
external sheet which pushes in below the notochord, and cuts off that structure from the 
mesenteron (PL VII. fig. 6). These mesoblastic cells do not become a fibrous layer for 
some time, but later they give origin to the muscles of the canal and its connective-tissue, 
while externally they give rise to the epithelial peritoneal layer. 

In the oesophageal region the course of the hypoblastic cells is extremely difficult to 
follow. They give origin to a cardiac swelling which is sub-oral and median (hr, PL IV. 
fig. 13; PL V. fig. 8), while other cells pass into the hypoblast laterally to form the 
core of the visceral folds. During the first few days after hatching the anus is still 
undifferentiated, as Lereboullet found to be the case in Perca ; nor is the oral cavity 
externally open, as the same observer also proved in Perca by experiments with various 
colouring matters (e.g., indigo), the alimentary tract being in fact a closed cylinder, con- 
sisting of a very thick inner wall of cylindrical cells (hy, PL IV. fig. 11), whose free 
rounded ends project into the cavity of the gut (fg), and externally of a thin layer of 
flattened mesoblastic cells (sp) not yet transformed into muscular and other tissues (vide 
No. 93, p. 625). 

Many preparations show a lining apparently of cilia,* and there is thus great probability 
that the enteric tract — the oesophageal portion at least — of young Teleosteans is ciliated. 
Its walls for some time are straight and smooth, but in later stages folds and wrinkles are 
formed, the intestine especially showing a complexly folded internal surface (PL XIV. fig. 5 ; 
PL XVIII. figs. 1, 11). The various parts of the tract become rudely marked during the 
first week after hatching. Thus a gurnard on the thirteenth day (PL VII. fig. 9) shows 
very distinctly a capacious though depressed oral chamber, the floor of which is ridged by 
the branchial bars and hyoidean framework, followed by a wide oesophagus (fg), the lumen 
of which is so flattened as to be little more than a horizontal fissure in transverse section. 
From this portion the duct of the swim-bladder passes (PL VII. fig. 4).t The enlarged 

* Shipley has recently described the oesophagus in Petromyzon (47th day) as ciliated (No. 150, p. 351). 
t Vide the highly suggestive remarks of Prof. Cleland on Teleostean pneumatic ducts — Memoirs, &c, in 
Anatomy, 1889, p. 170. 


stomach (st) follows, and beneath its thin walls the hepatic mass lies. A fourth portion 
of the tract succeeds, viz., the pyloric section, the dense walls of which give origin to those 
remarkable diverticula, the pyloric cceca. These seem to be merely blind evaginations, and 
gradually assume a lanceolate form, as we find in young cod from \ to 1^ inch in length. 
Ventrally a well-marked duct passes from the liver, viz., the ductus choledochus, with 
several ramose biliary ducts. The intestinal walls are very dense, rapidly develop a glandu- 
lar character, and have a narrow oval lumen (hg) with local enlargements, especially in the 
mid portion of the gut. Posteriorly it narrows again until the rectal region is reached, 
where a cincture or valve occurs, behind which its capacity once more enlarges (see 
also hg, fig. 8 on the same plate) ; it then bends downward, and narrows to form the small 
anal aperture (a) opening upon a muscular papilla. A similar condition of the intestine 
and rectum is seen in the figure of P. platessa (PL XIV. fig. 5). The rugose walls often 
exhibit vermicular movements, which are, however, very irregular, and involve and pro- 
duce great contortions in the alimentary tract ; thus a peristaltic motion may pass from 
the mesenteron to the rectum, narrowing its capacity as though by an embracing cincture. 
Mouth. — A stomodseum or involution of epiblast to form the mouth is never really 
formed in pelagic Teleosteans.* The oral cavity is capacious, and the branchial frame- 
work supporting its floor and sides is fairly advanced when a fine transverse fissure is 
seen passing across the under surface of the head below the eyes (m, PL IX. fig. 2). 
This fissure enlarges and lengthens, forming an almoncT-shaped opening (m, PL IX. 
fig. 3) across the subcephalic membrane. This is the mouth, and it is formed as a slit by 
the lumen of the buccal chamber bursting through. Its edges are jagged, and strands of 
cellular tissue often pass across from one lip to the other one or two days after the oral 
opening appears (PL IX. fig. 3), showing that it is an actual severance of a complete 
epiblastic membrane. There is no indication of the double origin, the coalescence of two 
lateral clefts which Dohrn has described in Gobius, Belone, and Hippocampus (No. 52a) ; 
but in the ling — the species illustrated in the figures just referred to, and in other forms 
— this median transverse fissure suddenly appears, and in the course of two or three days 
widens antero-posteriorly to form a large median tubular opening, t The lips do not 
move, but the hyoid cartilages are flexible and mobile, and the floor of the mouth is thus 
raised and depressed. The mandibular cartilages rapidly grow forward, and the oral 
opening — at first ventral, transverse, and shark-like — assumes the shape found in the 
adult Teleostean, the prolongation of the mandible not only bringing forward the aperture 
of the mouth (PL XII. figs. 2, 6, 7), but proceeding so fast and to such a degree that the 
floor actually extends beyond the snout, and the aperture now opens from above 
(PL X. figs. 1, 2, 3, 5, 5a). The suborbital curtains, which haug down like two mem- 
branous flaps, diminish, and become denser on account of the development in their tissue 
of maxillary bars, the chitinous character and form of which are elsewhere described. 

* Parker describes a true stomodseuni in the salmon, but probably his account of the ingrowth of epiblast to 
form the mucous membrane of the mouth and fauces requires confirmation. 

t Dohrn, on the contrary, describes the centre of the oral slit as still closed when the lateral portions have broken 


Anus. — The anus in the forms here described is not a proctodseum, as it is not 
produced by the ingrowth of the external epiblast, but is at first a lateral opening (see 
PL VII. figs. 12-15), which five or six days after hatching is formed by the protrusion 
of the anal section of the alimentary canal. In Molva vulgaris, early on the second day 
after emerging, the anal tract seems still to end blindly, being continued backward 
nearly in a straight line, or in some cases sending down a terminal process at right angles 
to the main axis of the canal. This terminal prolongation is carried down to the middle 
of the marginal fin, and generally on the second or third day is found to break through 
in a manner not unlike the oral opening. The rectum is thus a capacious thick-walled 
tube, sending out a narrow anal continuation consisting of a fine tube lined by a single 
layer of cubical epithelium, and it passes through the thick tenacious plasma contained in 
the space behind the urinary vesicle (PL VII. figs. 12, 13). This space is enclosed between 
the two epiblastic lamellae of the caudal membrane, and the anal tube curves round and 
opens laterally on the surface of the latter, some distance from the ventral margin. 
Later the membrane below the aperture becomes absorbed, the rectum assumes thicker 
walls {Jig, PL VII. figs. 8, 9), and the usual muscular rectal portion of the alimentary 
canal is formed during the second week after emerging. The anus then opens in the 
ventral middle line, as in the adult fish. 

Liver. — Soon after the otocysts are formed the ventral wall of the mesenteron in its 
fore part shows an enlargement — " an ovoid dilatation just before and below the early 
pectorals," according to Lereboullet (No. 93, p. 584), and his description holds to a 
large degree for pelagic Teleosteans. Certainly the liver is a distinct outgrowth from 
the ventral wall of the mesenteron. Hoffman has expressed the view that the liver 
originates from the yolk-periblast, and that the hepatic diverticulum is really a prolifera- 
tion of " parablast entoderm" (No. 69a). Such sections as fig. 2, PL VII., do not 
support this view, for the periblast (per) is a distinct, granular layer beneath, and 
separated by a delicate stratum of hypoblast (Jiy) from the cells which build up the liver. 
The liver, in fact, is largely a solid proliferation of the ventral wall of the mesenteron, and 
is periblastic, or formed of " parablast entoderm " only in the degree that the ventral wall 
of this region is periblastic, and this we have seen at this point to be at a minimum. 
Into the early liver (Ir, PL VII. fig. 5) a delicate canal (dc) passes, a direct prolongation 
of the enteric lumen, doubtless the ductus choledochus. Lereboullet noticed this 
especially when the mesenteron dilated and contracted as it does in later embryonic 
stages (No. 93, p. 593). In Perca, on the sixth day, the same observer describes 
numerous ramifying fissures or prolongations from this delicate canal; and the gall- 
bladder he regards also as a tubular outgrowth of the intestine. The hepatic pro- 
liferation becomes bifid, a dorsal and a sinistral ventral lobe being distinguishable. The 
liver also becomes divided into small lobuli {J/r, PL VII. figs. 1-3), in the midst of which 
the spacious gall-bladder (gb) appears as a clear vesicle, limited by an epithelial wall 
of a single layer of cells. 

Swim-Bladder. — From the dorsal wall of the mesenteron (mg) the swim-bladder (sb) 


is given off as a very thick-walled diverticulum (PL VII. fig. 5), which presses upwards 
against the notochord, and remains connected for some time by a fine canal (PL VII. 
figs. 2, 4). Before the embryonic period ends, however, the duct atrophies, all the forms 
specially referred to being physoclistous. 

Heart and Circulation. — The heart is developed at a very early stage — before the 
oesophagus is formed — as a cylindrical structure (hr, PL IV. figs. 8, 12), in front of the 
pectoral region, i.e., between the otocysts and the optic vesicles. Soon after the alimentary 
tract is defined, or, as Wenckebach expresses it (in the case of Belone), after the ventral 
closure of the gut, and when about twenty-four proto-vertebrse are marked off, the heart has 
a vermiform shape, and is still solid. This solid condition Lereboullet described in Perca, 
but in Salmo and other forms the heart is stated to appear in the form of a single or 
double tube (vide Hoffman, No. 69a,; Balfour, No. 11, p. 637). That the heart 
develops as a single tube in the Gadoid and other forms here considered is not surprising. 
When the heart arises as two tubes it appears to be connected, as Balfour pointed out 
(No. 15, vol. xi. p. 689), with the non-closure of the pharynx inferiorly, but in those 
Teleosteans where the oesophageal cavity is formed later by a forward growth of the enteric 
lumen, the solid tract is really closed below, and this is the condition correlated with an 
unpaired cardiac rudiment. Its first indication in the living embryo is seen as a rounded 
projection beneath the solid oesophagus bulging out towards the subjacent periblast. 
It is a ventral outgrowth of that splanchnic mesoblast, winch also forms the branchial 
arches. Lereboullet describes this cardiac swelling (on the seventh day in Perca) as 
having its inferior portion, the auricle, resting directly on the yolk (No. 93, p. 584; vide 
his pi. iii. fig. 13). It is a median unpaired projection, and carries down before it a 
very thin layer of hypoblast. At times this delicate stratum of hypoblast cannot be 
made out, and in P. platessa it would appear to be absent; nor can a layer of hypoblast 
be distinguished over the rest of the surface of the yolk, though such a layer is readily 
seen in other Pleuronectids, as well as in Gadoids (PL VIII. fig. 11). In all cases, however, 
the continuity of the rudimentary heart and the "branchial" mesoblast above is 
maintained.* Hoffman describes in Salmo two lateral folds of splanchnic mesoblast, 
which pass down beneath the pharynx, and produce by a dorsal and a ventral union a 
tubular heart (No. 69a ; vide fig. 9, Taf. ii.). Before the tube is complete inferiorly, some 
intruding cells of " parablastic entoderm," i.e., periblast, form the cardiac endothelium 
(No. 69a, Taf. iii. fig. 6; Taf. iv. fig. 6). Such a process does not accord with the appear- 
ance of the heart in the living condition, for in the embryonic cod, haddock, and others 
no lumen is visible at first, as Oellacher and Gotte also hold, and indeed after the lumen 
is formed the endothelial lining is absent (vide surface- views, PL VIII. figs. 3, 7; and 

* Wenckebach (op. cit, and Jour. Roy. Micr. Soc, February 1887) describes its first appearance as a band of meso- 
dermic cells close behind the optic vesicle on the lower surface. They arise from the indifferent rnesodermie cells of the 
head which wander round the gut. The mass of cells splits to form a kind of pouch — the heart. The blood-vessels 
have a similar rnesodermie origin. The heart opens into the segmentation-cavity, and its lumen is nothing else than 
part of the blastoccel. The blood is rnesodermie in origin, he avers, neither endoderm nor free periblast, i.e., nuclei, 
having any share in its formation. 


sections, PI. XI. figs. 2, 3), indicating that the epithelioid layer is not formed in some 
Teleosteans simultaneously with the formation of the cardiac tube, and favouring the 
view that the heart becomes tubular by dehiscence of its median cells, or, as Lereboullet 
says, the linear cavity is formed partly by separation of cells and partly by absorption 
(No. 93, p. 551). * It seems probable that in different Teleosteans this organ has a 
different structure primarily, and certainly at later stages the circulatory system diverges 
in various species. Thus in the Gadoids, Pleuronectids, Trigla, and other pelagic forms, 
no yolk- circulation is ever developed, whereas in most demersal forms a circulation upon 
the surface of the yolk is a very striking feature, and may be said to a certain extent to 
precede the heart's action ; for Truman found in Esox that blood-corpuscles were formed 
in patches in the cortex of the yolk, constituting the " islands of blood-corpuscles" which 
Gensch has described (No. 56), and that before the heart pulsates, blood actually moves 
towards that organ. At the eighty-sixth hour Truman saw these moving corpuscles reach 
the heart, but it was ten or twelve hours later before the organ exhibited any motion, and 
even then no corpuscles passed into its cavity (No. 154, p. 191) ; so that the pulsations 
are independent of any stimulus given by the presence of blood-corpuscles within its 
chambers. Muscular twitchings, again, are often observed in the heart of the gurnard 
before the proper pulsations begin. We have already seen that the cardiac chamber is 
enlarged by the raising of the head of the embryo, and Lereboullet noticed that as this 
took place in Perca the heart becomes detached from the head, its anterior end following 
the retreat of the yolk, sinking slowly, while the hind end remains attached under the 
embryo. While yet a simple tube, the heart is contractile, the early pulsations, which 
commence usually one or two days after the heart is formed, being one of the most note- 
worthy features in the developing embryo, though no haemal fluid can be made out.t At 
first the pulsations are very slow and intermittent, the intervals between the contractions 
being irregular. In an embryo, four days after fertilisation, the beats are more rapid and 
regular, averaging 48 pulsations per minute, while the rate at times is greatly increased. 
Thus Dr Truman found in Esox, soon after the heart began to beat (at the ninety- 
ninth hour), they reached 104 per minute (No. 154, p. 193), but the conditions 
must have been abnormal. The rate noted by Lereboullet in Perca, viz., 40, 50, or 
60 times per minute, is normal (No. 93, p. 451). In a ling of the second day (PL XIII. 
fig. 4) the pulsations were observed to have reached the rate of 80 beats per minute. 
The endothelial lining of the heart appears as a single delicate layer of cells, very much 
flattened and loosely suspended in the cardiac chamber, apparently derived from the 
myocardium or thick contractile layer. Oellacher regards it as developed in the trout 
from the hypoblast beneath, and his figures on Taf. iv. (No. 114) are very clear; but 
no such continuity of the endocardium with the limiting hypoblast below is shown in 

* In certain insects Patten has found that two mesoderinic plates by a median longitudinal fusion form a solid 
cord (Phryganida), while in others (Blatta) it is hollow from the time of its formation, and the mesodermic folds pulsate 
long before they unite to form the heart (Patten, "Develop, of Phryganids," Quart. Jour. Micr. Sci., vol. xxiv., 1884, 
pp. 587, 597). 

t Trawling Report, 1884. 


sections of our forms. Certainly it is not formed, as has been suggested for the chick, 
by wandering corpuscles from the area vasculosa, which, finding access to the heart, cling 
to its walls as a loose lining, for in Teleosteans this endocardium is present before the 
haemal circulation is in action. Hoffman's figures (No. 69a, Taf. iii. fig. 9, Taf. iv. fig. 6) 
do not represent the primary condition in our forms, for the heart usually pushes down 
before it a delicate stratum of hypoblastic cells (hyp, PL VIII. fig. 11; PL XL fig. 2) ; 
but this limiting ventral layer apparently becomes obliterated anteriorly, and the 
pericardial chamber is open to the subembryonic space, which is undoubtedly the persist- 
ing germinal cavity. The vermiform outline (h, PL VIII. fig. 3 ; PL XII. fig. 4) soon 
undergoes modification, and the posterior end becomes expanded, while the anterior and 
upper ventricular portion remains narrow (h, PL XIV. fig. 1). Thus the simple cardiac 
tube becomes cone-shaped, the apex of the cone being continuous with the sub-cesophageal 
mesoblast (nies, PL VIII. fig. 11), while the lower anterior end is comparatively free, 
though not perfectly so, as a thin mesoblastic membrane (PL VIII. fig. 5) continuous 
with the free edge of the auricle separates the myocardium from the exterior, and a space 
is formed — the pericardial chamber (pd) around the heart. 

By its mode of formation as a downward growth the heart has at first a somewhat 
vertical position ; but with its increase in length it extends further and further forward 
beneath the head, and moreover it becomes flexed to the right (PL VIII. figs. 2, 9). The 
anterior position of the heart at this time is quite characteristic of the early embryo. 
The before-mentioned delicate pericardial walls are involved in the rhythmic movements 
of the organ, and sway to and fro with each systole and diastole. 

The splanchnic mesoblast, out of which the heart and pericardium are formed, has 
relations similar to the splanchnopleuric prolongation in the region of the trunk proper, 
— the pericardial cavity surrounding the heart just as the cceloma encloses the abdominal 
viscera, — the view that the former is merely a part separated off from the latter by the 
posterior (pericardial) septum being strikingly supported by the condition in the 
Cyclostomes, in which an intercommunication of pericardium and body-cavity persists 
throughout life. 

The first change in the position of the slightly curved cylindrical heart (PL VIII. 
fig. 5) results in its assuming an L-shaped form (as in PL IX. fig. 1, h), the small arterial 
end (ventricle, ven) still occupying the median position, while the auricular end (aur) is 
turned at right angles. In the figure before referred to, the flexure is still more apparent ; 
while in fig. 3, PL XII., the auricle, previously directed to the front (PL VIII. figs. 3, 6, 8), 
is now posterior (see also PL VIII. fig. 9), the flexure continuing to increase as development 
proceeds. Thus the relations of the auricle and ventricle are reversed, and the latter, 
which is now anterior, becomes bulbous (ven, PL VIII. fig. 7), and distinctly marked off 
by a constriction ; while the auricle (au) itself is separated by a cincture into auricle 
proper and sinus venosus (sv). The blind continuation of the ventricle into the sub- 
pharyngeal mesoblast (mes, PL VIII. fig. 11) above is really the rudimentary bulbus 
arteriosus, so that the four parts may be distinguished, as Ryder pointed out (No. 141, 

VOL. XXXV. PART III. (NO. 19). 6 G 


p. 537), about or soon after the time of hatching. Occasionally one or more detached cells 
may be seen loosely suspended in the auricle, near its external opening, and they swing 
to and fro with the heart's pulsations. No blood, as such, exists until a later stage, and 
any fluid included in the lumen of the heart and the pericardium must be non-corpus- 
culated, and its presence cannot be demonstrated. It may be doubted whether the stray 
corpuscles above referred to are blood-elements at all, for Lereboullet is almost certainly 
correct when he says that it is erroneous to assert that the corpuscles which first appear 
in the heart are detached from its walls: "they are different in character, and too 
coherent to become detached" (No. 93, p. 585). In our specimens these had the appearance 
of papillae on the cardiac wall. Ryder regards the periblast as the source of the blood- 
corpuscles, in accordance with Hoffman (Zool. Anz., 1880, p. 633) ; and in this view 
their connection with the so-called free nuclei around and beneath the early blastoderm 
is naturally suggested. Eyder contends (No. 141, p. 537) that the pericardial cavity 
is really the persisting segmentation- (or more correctly, germinal) cavity, and that the 
passage of periblastic blood-elements into the heart is thus secured. It must be 
remembered, however, that the roof of the germinal cavity consists of the subembryonic 
hypoblast, a layer which stretches beyond the tip of the snout of the young fish, and 
extends as the under-stratum of the double-layered yolk-sac (ys, PL V. fig. 8). This 
subcephalic chamber, with its floor of periblast and roof of hypoblast, is never obliterated ; 
but though its periblastic floor does not bud off cells to form the ventral half of the 
mesenteric wall, yet its roof (ys) becomes pushed downward (vide PL XII. figs. 1, 3) 
until it lies below the pericardium (PL XII. fig. 2 ; also see PL VIII. fig. 6), and is 
separated only by a narrow fissure from the periblast (per) beneath. The germinal 
cavity diminishes in a less degree laterally, and the latero-pharyngeal spaces into which 
the embryonic breathing aperture opens from without (see p. 747) are its more visible 
remnants (ss, PL IV. fig. 21 ; PL XL figs. 6, 7, 8). The floor of the pericardium 
appears (vide PL IV. fig. 21) to be obliterated anteriorly, but even in this case 
the delicate hypoblast would seem still to separate the pericardial from the germinal 
cavity below. The absence of the limiting layer from a certain area may be explained 
also, not by obliteration, but by a different method of origin, and it is quite possible that 
the pericardium may be a fold of mesoblast directed forward. Truman, indeed, speaks 
of such a mode of development in Esox, a membranous fold being reflected from the 
under part of the head (No. 154, p. 190). 

Meanwhile the vascular canals of the trunk are in course of formation, a small arterial 
vessel (the dorsal aorta) being hollowed out of the loose trabecular tissue (really the 
intruding mesoblastic cells above the gut which are broken down) along the under side of 
the notochord (x, PL VII. figs. 1, 4, 6 ; PL XII. fig. 8), and two venous trunks of large 
calibre are similarly formed in the lateral connective tissue just external to each head- 
kidney. In the living larva of Molva vulgaris, on the fourth day, the subnotochordal 
tissue seems to be traversed by a single large vascular channel (vn) separated by an 
interval, probably the dorsal aorta (ao), from the chorda (nc). The large channel can 


be traced from the liver posteriorly to the caudal region, and it contains numerous large 
round corpuscles, though these do not seem to occur anterior to the liver (PL XV. fig. 1). 
Again the venous trunks immediately in front of the pectoral fins send prolongations 
downward, and communicate with the venous end of the heart, which at this time shows 
the broad auricle directed upward and backward, and a spacious sinus venosus (sv, 
PL XII. fig. 8). The large venous tube thus passing to the sinus on each side is the ductus 
Cuvieri, which, in addition to the posterior (cardinal) vein, also receives the anterior 
(jugular) trunk. Around the two anterior veins the cellular tissue of the pronephros 
grows (prn, PL XL figs. 9, 11), and venous ramifications are developed in the midst of 
the renal matrix. Before the end of the first week after hatching — generally on the 
fourth or fifth day in Gadoids — a simple circulation can be detected. The anterior 
bulbous end of the heart driving the blood upwards behind the eyes — probably by the 
artery of the hyoid arch, whence it courses by the great subnotochordal trunk (dorsal 
aorta) to a point a little posterior to the root of the tail, and, passing round by a minute 
loop, returns by a large venous trunk which anteriorly divides into the two cardinals 
already mentioned. The two subnotochordal trunks with the anterior branchial artery 
constitute the simple vascular system in its earliest condition. A day or two later a 
venous branch leaves the vena vertebralis at a point about midway along the trunk — 
above the mesenteron, and passes down to the lower side of the alimentary canal — and 
forward along the margin of the liver to the sinus venosus. This must be the 
subintestinal vein, which is, however, usually described as passing along the intestinal 
portion of the alimentary canal. Its course, however, is at this stage very short. Not 
so in the case of the intestinal artery (coeliaco-mesenteric) which leaves the dorsal 
aorta in the pectoral region, traverses the mesenteron in descending, then courses 
beneath the rectum, but ascends before reaching the anus, and passes along the anterior 
margin of the urinary vesicle to join the caudal vein. The caudal vein is lengthening 
simultaneously with the caudal artery ; thus, in a cod on the seventh day both reached 
barely a quarter the length of the caudal trunk, while on the fourteenth day they 
extended almost to the tip of the tail. The force of the arterial current seems to hollow 
out the yielding channel, and causes it to become longer, but the afferent venous trunk 
has the appearance of a somewhat irregular ill-defined sinus. During the second week 
great advances take place in the haemal circulation. Lereboullet noticed, in Perca 
about two weeks old, that blood was passing along two of the gill-arches ; and in the 
Gadoids and other forms described in this paper two arches likewise develop arterial 
channels. There is considerable variation in the details of this development ; thus a 
haddock, on the fourteenth day after extrusion, showed arterial blood passing along 
two (apparently the anterior) branchial arches, whereas another embryo of the same 
species, on the eleventh day, showed three branchial arteries and a fourth artery, which 
runs apparently within the opercular fold, possibly, however, the hyo-opercular. The 
mandibular artery is a well-marked trunk coursing along the outer margin of the 


mandible.* Both arteries meet in front of the symphysis, and return by a single median 
vein along the floor of the mouth. The later developments of the haemal system at a 
stage — in, say, Gadus morrhua — when the caudal artery extends along fully two-thirds 
the length of the tail, are as follow : — Four branchial arteries oan be made out, and 
a submaxillary artery passes beneath the eyes, while a return-current is directed over 
the e} 7 es, along the supraocular vein. The cceliac artery, before described as leaving 
the aorta in the pectoral region, passes over the liver, along the ventral surface of the 
intestine, and sends an arterial branch upward, which, bifurcating, supplies the walls of 
the intestine, — the main trunk continuing its ventral course, and ascending in front of the 
urinary vesicle, — over the walls of which it passes to the vena vertebralis. The venous 
trunks form a more complex system — the simple subintestinal loop which breaks up into 
an elaborate hepatic capillary network still continues, but it is joined by a large visceral 
trunk on the posterior side of the liver. This latter vessel leaves the caudal vein at the 
root of the tail, passes ventrally in front of the urinary vesicle and over the walls of the 
rectal portion of the intestine to the termination of the mid gut. At this point a 
large venous trunk branches off dorsally to join the posterior cardinals. Minor venous 
branches run from the walls of the stomach and pyloric portion of the intestine, forming 
the first indication of the portal system — all their blood finally passing in front of the 
liver into the sinus venosus by the hepatic veins. The liver, the dorsal lobe of which 
lies above the alimentary canal and behind the swim-bladder, is seen chiefly as a rounded 
mass (the left and ventral lobe) projecting boldly into the surface of the yolk below, and 
lying immediately in contact with the posterior pericardial wall. The proximity of the 
liver with its rich vascular plexus, and of the large ductus Cuvieri pouring a stream into 
the capacious sinus, suggest the possibility that it is at this point that the assimilation 
of yolk-matter is most active. It is absorbed and conveyed to the heart by the venous 
blood. The continuity of the wall, limiting the pericardial chamber (pd, PI. VII. fig. 9), 
appears to be unbroken, and roofs over a sub-pericardial space (ss) filled with a serous plasma 
and disintegrated yolk. A suboral chamber in many cases seems also to be shut off by 
this membrane (PL VIII. figs. 6, 7). The heart's pulsations partake of a progressive 
vermiform movement, the auricle, continuous with the sinus venosus, contracting first, 
and the successive parts (of the auricle) contract in order, the ventricle dilating as the last 
part of the auricle closes. As the ventricle contracts, the open end of the auricle dilates. 

The progressive systole being triple, . .12 3 

/a • r \ /a\ /-d\ {C\ contract 

(See accompanying diagram,) . . . (A)— (B)— ^J dilateg 

The diastole also is threefold, and D contracts ) (A) — (B) — (C) dilate 
simultaneously with the dilatation of A, . j (D) contracts. 

The delicate pelagic forms chiefly considered in these pages present a great contrast to 

* Gotte is certainly incorrect, as Balfour pointed out (No. 11, p. 645), in denying that a mandibular artery is 
ever developed in Teleostei (No. 59). 


the stronger and more robust Teleosteans, which are at a very early stage, often long before 
extrusion from the egg, provided with a complex vitelline circulation. In such forms as 
Salmo (PI. XXII. figs. 4-9), Anarrhichas (PL XX. figs. 2, 4, 5), Gastrosteus, Cottus, 
Liparis (PI. XV. fig. 2), and Cyclopterus, the blood-corpuscles seem to be mainly 
derived from the nucleated particles into which the surface of the yolk becomes broken 
up, and, as already noted, Truman found in Esox that haemal channels appeared upon 
the yolk, and corpuscles slowly moved towards the heart before this organ showed any 
motion. No such blood-canals are excavated in the yolk of the pelagic forms here 
treated of, indeed no yolk-circulation ever truly exists in the gurnard, cod, and allied 
forms. Nevertheless, the yolk steadily diminishes, and in embryos, fourteen to twenty 
days after hatching, it forms but a very slight projection, and at the end of the first 
month would appear to be entirely absorbed (compare fig. 5, PL XIX. and fig. 1, 
PL XVI.). The surface of the yolk, however, shows during this time rapid disintegration 
[vide PL VII. fig. 9), vesicles, granules, and nucleated particles appear in it (PL XI. 
fig. 12), and are especially noticeable around the large oleaginous spheres (PL XL 
fig. 13) in those forms, such as the gurnard, ling, and others, in which these striking 
bodies occur. The protoplasmic envelope of the globule in such cases becomes richly 
provided with large nuclei showing one or more nucleoli, and similar bodies occur 
superficially over the yolk. In a young perch, eleven to fourteen days old, Lereboullet 
observed, just as we have noticed in the Gadoids and other forms, the dorsal aorta, formed 
by the union of the vessels of the branchial arches, sending a supply to the intestine and 
adjacent viscera, and reaching to the extremity of the tail, while of venous trunks the 
two anterior and two posterior cardinals and the subintestinal vein are common to both. 
In Perca, in addition to the above trunks — developed no doubt in all Teleostean larva?, 
a complex yolk-circulation arises, and is supplied by branches from the posterior cardinals 
and from the subintestinal vein. These branches pass over the yolk as simple undulating 
lacunae formed by the separation of the substance of the yolk-cortex, and meet on the 
ventral side of the yolk in a pair of large veins, which form one large sinus, continuous 
with the sinus venosus in the pericardial chamber. Lereboullet says of these vitelline 
vessels, that they do not appear to have proper walls, and form an ill-defined and irregular 
network; but on the third or fourth day after hatching the haemal canals acquire definite 
walls, the network elongates, so that the main trunks show a parallel arrangement (No. 93, 
p. 601). In Perca the development of this circulation over the yolk is much more rapid 
than in Esox, and Lereboullet connects this with the larger perivitelline space in Perca, 
as there is a greater need for respiration; and for this reason, he says, in that species " the 
capsule is spacious, and holds so large a quantity of water" (No. 93, p. 610). The true 
explanation, however, seems to be that the more complex and rapid the circulation the 
more speedily the bulk of the yolk is reduced, and hence a large perivitelline space is 
produced. It is remarkable, however, that in such forms as the gurnard, rockling, the 
flat fishes, and Gadoids, in which no vitelline circulation ever develops, the yolk should 
still show a very rapid disintegration (compare PL XII. figs. 1 and 3, with PL X. 


figs. 1, 2, and 3, and PL XVII. fig. 2). This does not take place, however, to any very 
appreciable extent while the embryo is within the ovum, whereas the reduction is 
very marked in Perca (No. 93, p. 610), Cyclopterus, and similar species. After the 
embryo emerges in pelagic forms, and before any circulation of a corpusculated haemal 
fluid exists, the yolk, which is very large and prominent in the newly hatched fish, 
becomes speedily diminished. A process of absorption must be actively going on in 
these forms (e.g., cod), and the presence of a transparent plasma bathing the tissues, and 
filling the pulsating heart and lacunae of the trunk, is suggested. 

The origin of the blood-corpuscles is an interesting point ; but there is little 
unanimity amongst observers on this matter respecting Teleosteans, and appearances seem 
to support more than one suggested mode of origin. Ryder, with Hoffman and others, 
as we have already said, holds " that the blood-cells are budded off directly " from the 
periblast, the nuclei of which layer by division give rise to groups of granules, the form- 
elements of the blood (No. 141, p. 543). C. Vogt in 1842 distinguished a " couch e 
haematogene" (No. 155), as did also Rathke and Von Baer, their third or vascular layer 
of the blastoderm being, however, derived from the "lower layer" or hypoblast-cells ; 
and Van Bambeke, while admitting that the periblast or " intermediary layer " has not 
been proved to be this " vascular layer," appears to consider their homology very probable 
(No. 20a, p. 9). Gensch's researches support this view, the corpuscles arising from the 
layer surrounding the yolk — " Kupffer's secondary entoderm." In opposition to 
Kupffer's affirmation that the outer mesodermal yolk-sac gives origin to the corpuscles, 
Gensch found that in Esox and Zoarces viviparus no mesoblast was present in the region 
where they arose, the two-layered epiblast lying upon the granular periblast in which cells 
were imbedded. These cells give out pseudopodial processes, which are constricted off to 
form corpuscles, and these by subdivision produce blood-islands (vide No. 56). In Salmo, 
Alosa (No. 141, p. 537), Gastrosteus (No. 122, p. 494), and other forms, the phenomenon 
described by Gensch has been observed, yet it is not conclusive that the primary 
corpuscles are derived from the " Dottersack." That the periblast contributes to the 
nutrient haemal fluid of the embryo there can be no question, but the point of chief 
moment is, whence are the primary corpuscles derived ? As Lereboullet long ago 
pointed out, the heart beats for some time before corpuscles appear in its lumen ; and he 
added that the haemal trunks too are formed, as in the gurnard, before the corpuscles 
(No. 93, p. 577). Wenckebach, however, holds that in the process of formation the 
blood-vessels give origin to the corpuscles, so that both originate contemporaneously. 
This observer concludes that the blood-corpuscles appear to him to arise in a solid mass 
of tissue in the region where the vena vcrtebralis is afterwards situated, the cells 
constituting this mass being carried away by a haemal plasma, and acquire the colour 
and character of blood-corpuscles subsequently (No. 157). The polyhedral cells which 
Wenckebach shows filling up the lumen of the subnotochordal vein (vide No. 157, 
pi. viii. figs. 2, 3, &c.) are also found, in section, to fill up the aortic trunk, and there 
is no reason why the derivation of these blood-cells should not be extended to all the 


haemal vessels. The fact, however, seems to be that the form-elements of the blood are 
for the most part derived from the periblast, the primary corpuscles alone being 
moulded apparently from the detached cells of the subnotochordal trunks. In those forms 
in which a vitelline circulation is developed, the removal of nucleated periblastic cells 
and the formation of sinuous lacunae (primary haemal trunks) has been repeatedly 
observed, and may almost be taken as established. In those without such a yolk-circula- 
tion (and to them reference is in these pages chiefly made), the periblast also is seen in 
sections to break up into similar particles, and these doubtless pass into the sinus venosus, 
though in what way is not decided. Certainly the liver and alimentary canal, as well 
as the pericardial chamber itself, are, as already pointed out, in intimate relation to the 
periblast beneath the embryonic-trunk (PI. VII. figs. 1, 2, 6, 9; also PL XII. fig. 8), and 
the transmission of detached periblastic elements into the circulatory plasma may be 
accomplished without difficulty. Eyder, in Salmo and Tylosurus, found such corpuscles 
in the pericardial chamber (No. 141, p. 537). This further consideration favours the 
latter derivation rather than the subnotochordal origin, viz., the rapid decrease in the 
volume of the yolk, even in those which have no yolk- circulation. In such forms the 
yolk protrudes as a very bulky appendage (y, PI. XIV. fig. 1), but shortly before, and 
especially after the blood-circulation is visible, it diminishes very rapidly (y, PL XVII. 
fig. 1). Now, if before the haemal fluid flows through its proper channels, it were 
deriving its corpuscles from the yolk, and still more, if with the further development of 
blood-vessels in the trunk a corresponding increase in the number of corpuscles takes 
place, the rapid disappearance of the yolk is readily accounted for. It is noteworthy, 
too, that while the subnotochordal trunks are the first to be developed, the formation of 
the subintestinal vein and cceliac artery quickly follows, and as these probably communi- 
cate with hepatic lacunae, the periblastic elements would find easy entrance into the vascular 
system of the embryo. These nucleated cells, which make their way into the haemal 
plasma, are originally colourless, and Lereboullet describes them as at first spherical, 
afterwards becoming flattened and elongated. They rapidly acquire the characteristic 
tint. In weak and sickly embryos the circulation is languid and the corpuscles few, a 
feature Lereboullet also noted (No. 93, pp. 581-2). In monsters, especially double 
embryos, the circulation presents interesting features, each having its own circulation, 
though receiving nourishment from a common yolk. Lereboullet instances the case of 
a trout (double monster) in which the artery divides into two vitelline trunks, each of 
the two returning as veins to the corresponding embryo ; while in another case of a 
double-headed embryo, which possessed two hearts, one alone received blood from the 
vitelline veins, the other heart received nothing (No. 94, p. 246). 

Renal Organs. — The differentiation of a renal tract takes place at a very early stage. 
We have seen that on each side of the notochord (PL IV. fig. 10) cuboid masses of 
mesoblast are serially marked off as protovertebrae (my) soon after the separation of the 
somatopleuric from the splanchnopleuric lamella. Just external to the protovertebrae, a 
little distance behind the otocysts, a rod of cells is budded off from the splanchnopleure 


in close proximity to the intermediate cell-mass. Lereboullet observed that in Perca 
this structure develops earliest posteriorly, for he failed to trace it anteriorly, though at 
a later stage, about the time of hatching, he was able to follow its whole course (No. 93, 
p. 633) anteriorly and posteriorly. In some species a fold is developed, not a solid rod. 
Eosenberg seems to have been the first to speak of it as a diverticulum from the 
somatopleure (No. 138), and Oellacher, Hoffman, and others have confirmed this view. 
Ryder asserts that " the development of the renal organs in different genera of Teleosteans 
differs greatly in detail" (No. 141, p. 533), and this would certainly appear to be so, for 
in Salmonoids, which the observers named chiefly investigated, the origin of these ducts 
as longitudinal diverticula pushed dorsally towards the epiblast, as a groove-like fold, in 
fact, of the peritoneal cells, has been clearly shown (see Oellacher, No. 114, fig. 18 2 , 
Taf. iv. ; Hoffman, No. 69a, Taf. iii. fig. 3). Yet in Gadoids and Pleuronectids it is by 
no means clear that this is the precise mode of origin. In the earliest condition yet 
observed in these pelagic forms a longitudinal blastema or solid cylinder is formed on the 
outer margin of the intermediate cell-mass, just as we find in the chick. Defined at 
first in the region of the mid-trunk, this blastema rapidly extends forward to the pectoral 
region, but posteriorly it develops more slowly and is ill defined. A lumen is formed by 
the radiate arrangement of its cells, which separate at their common point of junction, 
and it is now outlined throughout its whole length some days before the embryo emerges. 
In an ovum (haddock) of the ninth day these structures are very distinctly seen as a pair 
of simple ducts, with walls consisting of a single layer of columnar cells, and extending from 
the pectoral region to the root of the tail. Anteriorly each tube is folded upon itself, 
turns inward towards the notochord, and ends in a trumpet-shaped infundibular opening, 
a condition exactly according with that described by Balfour and Parker in Lepidosteus 
(No. 18, p. 415) ; but in that species the authors agree with Rosenberg and Oellacher, 
that it is a hollow outgrowth of the somatopleure, and freely communicates with the body- 
cavity. The two ducts are widely separated, but as they pass backward gradually approach, 
and, curving down in the anal region, they meet and unite beneath the notochord in an 
unpaired common portion (uv, PI. VII. fig. 8, and in section fig. 6a), which is originally 
of small capacity and provided with thick walls. At first the ducts are somewhat super- 
ficial (prn, PL VII. figs. 1, 2, 3), as is implied in their mode of origin, being dorsally 
directed outgrowths of the proximal somatopleure ; but they undergo a change of position 
similar to that exemplified in the chick, and lie ventro-laterally to the notochord (sg, PI. 
VII. fig. 4), and ultimately protrude into the peritoneal cavity (sg, PI. XL fig. 14). 
Ryder did not make out the mode of termination in Gadus, and he supposed that the 
urinary vesicle opens either directly into a cloaca or the terminal portion of the intestine. 
The continuity of the walls of the ducts (sg) with the bilobed upper part of the urinary 
vesicle (uv) is clearly demonstrated in section (PI. VII. figs. 7, 11), and the urinary 
vesicle itself has an outlet in its early condition of an interesting nature. Lereboullet 
described in Perca the first condition of the ducts, and says that each must be a secreting 


organ solely, assuming the excretory function later, when the ovoid dilation (urinary 
vesicle) establishes a communication with the lumen of the enteron (No. 93, p. 633). 
Kupffer draws attention to a strand of cylindrical cells connecting this receptacle and 
the hind gut, " uniting," he says, " with the epithelium of the gut " (No. 87, p. 224); but 
he appears not to have made out, any more than Lereboullet, an actual communica- 
tion between the two. Yet such is the case. A distinct tubular connection exists ; but 
the walls of the vesicle (uv) as well as the enteron (hg) are extremely plastic and mobile, 
vermiform movements being frequent, so that the lumen between the two becomes wider or 
narrower, and at times appears to close up, though the communication is usually readily 
seen (PI. XX. fig. 13). Throughout their whole length, these excretory canals, including 
the urinary vesicle, exhibit simply a wall of nucleated cubical cells — a single layer of 
cylindrical epithelium. Such is the condition of the renal tract until the time of hatch- 
ing, viz., a pair of cylindrical tubes, which pass along each side of the subnotochordal 
hsernal trunks, to terminate, after curving inward and downward in an infundibular 
opening. In front of the crozier-shaped loop (pm, PI. XL fig. 11, and PL XXI. 
fig. 6) a mass of trabecular tissue lies, into which tubules appear to enter to some 
extent, but this loose connective is also penetrated from the front by the growing basilar 
plate. The simple character of the embryonic renal organs in the Teleostei may be taken 
as evidence of a primitive condition, in which no metamerism is seen, the simple duct, 
which is truly an archinephric duct, forming a loop in front, and communicating with the 
pleuroperitoneal cavity, while posteriorly it passes into the hind part — a cloacal section, 
in fact — of the enteric tract. 

During the greater part of embryonic life this simple condition continues, and the 
infundibular openings do not seem to increase in number; whereas in Amphibians several 
(three or four) are developed, and in Selachians they form a series. When the young 
fish emerges, the anterior end of the kidney shows signs of growing complexity, the folds 
of the loop increasing, and a vascular glomerulus being developed in front of the swim- 
bladder near each nephrostome. A little later the nephrostome of each side and its 
adjacent glomerulus are gradually enclosed in a capsule, this fibrous sac shutting off both 
structures from the general body-cavity. A section just behind the occipital region 
(PL XXVI. fig. 4) shows one of a pair of such capsules in the middle line and below the 
median hgemal trunks {ao and cv). On the lower and inner side of each capsule a 
vascular meshwork (gl) is present, while the nephrostome of the head-kidney opens on the 
outer side of the capsule. The rudiments of the single pair of glomeruli are seen in the 
newly emerged embryo, and are not fully developed until some days later ; but in 
Gastrosteus and like forms, which issue from the ovum in a more advanced condition, the 
later features are already exhibited. Ryder states that in Clupea alosa there is no 
evidence of the existence of a nephrostome or of the presence of median glomeruli until 
long after hatching (No. 141, p. 534), and this is certainly remarkable, though in the 
Gadoids and others great variations are observable, the renal organs being fairly advanced 
in P. platessa a day or two before hatching, whereas in P. fiesus and P. limanda they 

VOL. XXXV. PART III. (NO. 19). 6 H 


are more rudimentary. The waste-products taken along the renal ducts originally pass 
directly from the body-cavity, but they are by and by conveyed from the special excre- 
tory Malpighian capsules into the urinary vesicle behind, a condition which remains 
essentially unaltered in the adult. The archinephric duct does not really close early in em- 
bryonic life, as has been stated (No. 48, p. 13), but opens into a special closed part of 
the body-cavity. With the further development of the anal region, the unpaired enlarged 
portion into which the ducts pass posteriorly communicates not with the rectum some 
distance from the external orifice as in the figure before referred to, viz., PL XX. fig. 13, 
but by a special passage with separate opening posterior to the anus, as in a cod the 
third week after emerging — a condition also shown in the gurnard three weeks old 
(PL VII. fig. 9). Of the series of segmental tubules and glomeruli seen in Elasmobranchs 
there is no trace in Teleosteans ; but though the renal organs are so simple in these 
latter forms, the interpretation of the various parts is not devoid of uncertainty. 

Teleosteans, it is generally held, agree with Cyclostomes, Amphibians, and Ganoids in 
possessing a pronephros ; but, in all, it is a larval structure, and is supposed to disappear 
in the adult. We have seen that in the embryos of the Gadoids, flat fishes, and 
gurnards an anterior trabecular meshwork (x) lies in front of the archinephric duct, and 
that this duct itself exhibits a much convoluted fore end (prn, PL XL fig. 11), with a 
nephrostome communicating with a glomerulus. The mid-portion of the duct becomes 
more or less convoluted, while the posterior portion remains comparatively straight, 
though on its dorsal side a large development of cellular tissue and small sinuous tubules 
takes place at a late or post-larval stage (PL XXIII. fig. 2). 

In the adult we usually find an enlarged anterior paired structure, the head-kidney or 
pronephros succeeded by a pair of elongated bodies, indisputably renal, which are much 
swollen terminally, often united, and traversed on their ventro-lateral margins by a pair 
of excretory ducts. Balfour examined various species of Teleosteans in the adult 
condition, and came to the conclusion, in opposition to Kosenberg, that the so-called 
head-kidney is not truly renal, though he did not deny the persistence of the larval 
pronephros in the adult stage (No. 13, p. 15). In Osmerus eperlanus, Esox lucius, and 
Anguilla, the fore part of the renal mass consisted in the main of vascular lymphatic 
tissue, while the true kidney-substance extended posteriorly. In Lophius piscatorius, 
which, according to Hyrtl, possesses a head-kidney only, lymphatic tissue, traversed by 
tubules alone, was found. This lymphatic tissue may represent the convoluted enlarge- 
ment of the archinephric duct, or merely a compact agglomeration of the loose cellular 
tissue lying external to the ductus Cuvieri and cardinal veins. It would appear that 
the latter is, in a large degree, true, the fore part being more emphatically trabecular, 
while the hind part consists of degenerate kidney-substance, so that Balfour's view most 
probably represents the facts, viz., that the so-called head-kidney is really a large 
lymphatic gland, concerned in the production of blood or lymph-corpuscles, while the 
hind portion is a remnant of the embryonic head-kidney. Except for certain lymph- 
spaces in the caudal region, the lymphatic system is but feebly represented in fishes, and 


it is interesting to see a large glandular structure, such as the so-called head-kidney, 
which may be made out in early embryos, and which is from the first closely associated 
with the main haemal vessels of the trunk. The lymphatic system, with its plasma and 
leucocytes, is really intermediate between a venous and an arterial system, and is 
associated with the various serous membranes, pleural, peritoneal, pericardial, and others. 
It is not surprising that large lymphatic masses should occur so near the centre of the 
blood-system, and though Balfour was not inclined to regard them as parts of the 
true kidney at all, they cannot at any rate be regarded solely as degenerate pronephric 
structures. Weldon, in his brief but interesting paper on Bdellostoma (No. 156), 
suggests that such masses are represented in all vertebrates by the suprarenal bodies. 
In Bdellostoma the archinephric or segmental duct is separated from this anterior mass, 
though in some specimens, possibly younger, traces of the continuity of the two could be 
made out. In embryonic Teleosteans the continuity is very patent, and in the adult 
condition renal tubules still ramify amongst the lymphatic tissue, as Balfour found in 
Esox, Lophius, and Osmerus. In the last species a single tubule alone passes into the 
vascular lymphatic mass. It would appear, indeed, as if the embryonic pronephros in the 
process of degeneration were usurped by the antenephric lymphatic structures, the 
proximity of both favouring this, while the persistence of stray tubules in the posterior 
part indicates the pronephric portion. Grosglik's researches upon various adult 
Teleosteans (Cyprinus carpio, Esox lucius, Rhodeus amarus, Gastrosteus aculeatus) con- 
firm Balfour's view, as he found coexisting in the region of the head-kidney lymphatic 
tissue and remains of the atrophied pronephros surrounded to some extent by the cardinal 
vein, while some pronephric tubules still pierced the lymphatic meshwork (No. 60, 
pp. 605-611). Emery, however, maintains that the pronephros persists permanently in 
such as Fierasfer and Zoarces ; while in other forms, as Blennius, it is provided with 
glomeruli and tubules, and in Merlucius esculentus it presents the peculiar structure of 
the Wolffian body. In all it persists as a recognisable pronephros (No. 53a), a view which 
Hyrtl held ; while Rathke and Stannius concluded that in Cyprinus the head-kidney 
is degenerate, and bereft of tubules, a view now generally adopted. The segmental duct 
precedes the development of the Wolffian body, and cannot therefore be a mesonephric 
duct, as Balfour suggests (No. 11, p. 701); it is in fact a pronephric duct, or more 
truly it is archinephric, for the pronephros is secondarily developed as a convoluted 
anterior portion. It is possible that this duct may not represent the primitive condition, 
but rather a segmental canal bereft of its serial segmental tubules and nephrostomes, 
save the single infundibulum at its anterior termination.* The view generally accepted 
however, is that which interprets it as a primitive non-metameric renal duct. The ducts 
retain their simple tubular character in the adult condition, and pass along the latero- 
ventral margins of the fully-developed renal masses. In the last larval stages, within a 

* The fact, however, that some segmental tubes, consisting of nephrostome, capsule, and convolutions, develop in 
Elasmobranchs independently of the duct, and later connect by their originally blind end, may indicate that the serial 
condition is secondary. It illustrates at any rate their separation and independent coexistence, whatever the explana- 
tion may be. 


month after hatching, mesoblastic cells become aggregated along the whole dorsal extent 
of the two ducts, especially in the fore and hind regions, and they present a somewhat 
glandular character, minute sinuous tubules appearing in their midst, which pass down 
and open into the longitudinal ducts. Plate XXVI. fig. 3, shows this elongated renal 
mass of segmental tubules, and presents largely the features of the permanent renal bodies. 
Still better is the relation of the parts seen in the section (PI. XXV. fig. 3). The 
simple epithelial walls of the excretory ducts (sg) are fibrous and thickened, and become 
in fact the permanent ureters. Gegenbaur views the pronephros as the primitive excre- 
tory gland of the Chordata, whose place has been taken by the mesonephros, and we see 
that while the pronephric ducts persist the phylogenetic replacement of the pronephros by 
the Wolffian body is ontogenetically repeated. It is noteworthy that the segmental ducts 
become much convoluted along their course, but especialty in the fore-portion. What- 
ever this may signify, these primitive archinephric ducts are the same as those which in 
Elasmobranchs and others connect the serial segmental tubes, but in Teleosteans they do 
not appear to divide longitudinally into upper or Wolffian ducts and ventral generative 
canals. The connective tissue which surrounds the renal organs becomes deeply 
pigmented at a very early stage (PI. VII. figs. 1, 3, 4, and 7), the large black corpuscles 
continuing to increase until their structure in later embryonic stages becomes obscured 
on account of the profuse distribution of these bodies (vide PI. XVII. figs. 1 and 2, and PI. 
XXVI. figs. 3 and 4). The close connection of the early segmental ducts and the rudi- 
ments of the pectoral fin has been pointed out, and it is interesting to note that the 
black pigment, surrounding the renal organs at a later period, extends over and is con- 
tinuous with the pigment-layer which passes to the base of the developed fin. The wall 
of the urinary bladder at a subsequent stage presents a consistent connective-tissue layer 
(conn), lined with columnar epithelium (epith), which in the upper portion forms pro- 
minent folds (PI. XXV. fig. 5). These folds are continuous with the two excretory 
ducts, which, as formerly stated, open into the upper and anterior wall of the vesicle. 

The Integument and Embryonic Pigment. — Throughout embryonic life the in- 
tegument remains thin and transparent, so that the internal structure of the young fish 
is readily seen. No cilia can be detected upon it. As already pointed out, a flattened 
external layer or stratum corneum (ep, PL IV. figs. 5a-5d) is distinguished from the 
subjacent layer, the neurodermis (ne). Soon after the notochord is defined these two 
layers extend as a distinct integument, not only over the dorsum and flattened parietes 
of the embryo, but as a yolk-sac, over the vitelline globe (PI. VII. fig. 6). The 
neurodermis, later in embryonic life, consists of several layers of pulpy rounded 
cells, which gradually merge into the flattened epidermis above. The innermost part of 
the two-layered epidermis constitutes a stratum Malpighii, and from it apparently exudes 
a lymphatic plasma, which forms a distinct fluid layer (ss, PL VII. figs. 1, 3, 4, 6), such 
a cutaneous sub-layer being found in Amphioxus and the Cyclostomes, though separated 
from the epidermal layers by the dermis proper. In Teleosteans when the mesoblast 
extends beneath the epidermis, to form the cutis proper, such a separation will be also 


effected. There is, at an early stage, no true dermis beneath the Malpighian layer. 
Pouchet speaks of this subepidermal tissue as a soft variety of laminated tissue, having 
a very loose texture, and therefore little firmness (No. 119, p. 291), but in its earliest con- 
dition it is simply a soft semifluid stratum in which amorphous matter abundantly occurs. 
In this layer pigment develops (pt, PL IV. figs. 13, 20), and always appears as definite 
amorphous corpuscles, not a mere diffused solution. 

In different species the early coloration shows very distinctive features, the colour of 
the pigment and its distribution being, in fact, so striking as to afford aid in diagnosis. 

In some species the pigment is confined to the embryonic trunk (PL V. fig. 2) ; in 
others it extends over the extra-embryonic layer, i.e., the yolk-sac (PL XVI. figs. 2, 8). 
Certain forms, again, exhibit one kind of pigment (PL XVII. fig. 1; PL XIX. fig. 8) ; others 
show two or more colours in the larval stages (PL XVI. figs. 1, 3, 5-9). No generalisation 
can be made, for in the same genus closely allied species show great diversity in these 
respects. Usually the pigment occurs in the form of minute isolated spots scattered 
upon the dorsum, and visible within one or two days after the closure of the blastopore ; 
though it frequently forms superficial protuberances, evidently pushing out the epi- 
dermal stratum at certain points. The form of the corpuscles undergoes rapid changes ; 
thus in a larval cod under examination two spots at the anterior border of the liver were 
seen to be finely branched, but before a sketch could be completed they visibly altered, 
and presented a simple rounded aspect. 

In the cod (PL XIX. fig. 8) and haddock (PL XVII. fig. 1) black spots only occur. In 
the ova of the former species, seven days after fertilisation, these spots, amorphous or 
rounded in form, were scattered sparsely over the dorsum and lateral regions, but in a 
few days they multiplied and extended from the snout to the tip of the tail, without any 
regular disposition. In larvae of the cod, soon after emerging, however, a further change 
in the distribution of the pigment takes place, for the spots, which are now elaborately 
stellate, become aggregated in four distinct bands (PL XIX. fig. 8), two very dense 
broad bands — a pectoral and an abdominal — occurring on the trunk proper ; while 
the tail exhibits two less dense bands, and often indications of a third. The haddock 
never shows this regular series of dark bands, which seem to be so characteristic in the 
newly emerged cod. In the ova of the haddock on the eighth day (two days after 
closure of the blastopore), black spots are irregularly dotted over the dorso-lateral 
regions, and subsequent changes chiefly affect the number and form of the spots. A 
larva two days after emerging shows stellate spots of the most elaborate form, which 
send out complex ramifying processes. These spots appear on the cranial region, and 
very thickly in the post-otocystic and lateral regions of the trunk proper. Posteriorly 
they are chiefly confined to the lower half of the caudal trunk, only two or three large 
spots occurring above the level of the notochord. Occasionally one or two spots are 
seen to send processes into the fin-membrane. The whiting offers a great contrast to the 
foregoing Gadoids, since on the eighth day (three days after the closure of the blastopore) 
very faint yellow spots appear, and are thickly distributed over the entire trunk, including 


the tail. Not only so ; but the fin-membranes and the yolk-sac exhibit similar spots in 
abundance (PI. XVI. fig. 2). They are very pale, and unless carefully looked for, readily 
escape detection, but they are very characteristic of this fish, even in the late larval stages, 
the pale yellow, with a distinctive greenish tinge rendering them important for diag- 
nostic purposes. In this species one or more enucleate, elaborately stellate structures 
frequently exist on each side of the mid-mesenteric region. Sometimes five or six of these 
bodies appear upon the surface of the yolk near the trunk of the embryo. They have 
the form of a " bone-corpuscle," but they are not pigmented, and their nature and meaning 
are doubtful. In the ling, from the third to the fifth day (PI. XIX. fig. 9), while the 
blastopore is closing, neutral-tinted amorphous spots, apparently protoplasmic aggrega- 
tions, which send out pseudopodial processes, and thus acquire a rudely stellate form, occur 
over the yolk-surface (vide PL XIX. fig. 9). Two days later (when about thirty 
protovertebrse are segmented off) the trunk and fin-membranes are very richly supplied 
with yellow pigment of a bright canary- tint (PI. V. fig. 9). This consists of unbranched 
corpuscles, and extends also over the yolk-membrane. Black pigment likewise appears, 
a few rude spots at first behind the eyes, and similarly it is not confined to the trunk, 
stray stellate spots extending over the yolk-surface, and especially over the protoplasmic 
covering of the large oily sphere (og). On the trunk, from the otocysts (au) to the tip 
of the tail, a more or less regular linear series of stellate black spots passes, extending at 
times over the dorsum. In M- otella, as Mr Brook (No. 31, pi. ix. figs. 7, 8a; pi. x. 
figs. 10, 11) has shown, black pigment occurs in definite patches ; and after the embryo 
has emerged, this definite aggregation of the spots produces a very remarkable appear- 
ance (PI. XVII. fig. 2). 

In the few species of Pleuronectidse as yet investigated, certain common features are 
noticeable, viz., the general occurrence of yellowish pigment (vide PI. V. fig. 6 ; PL XVI. 
figs. 1, 3, 5, 6 ; PL XVIII. figs. 1, 2 ; PL XIX. fig. 5), and in later stages the presence 
of two distinct colours (PL V. fig. 6; PL XVI. figs. 1, 3, 5 ; PL XVIII. figs. 1, 2). 
On the fifth day (120th hour after fertilisation), when twenty-two to twenty-five 
protovertebrse in the common flounder are marked off, pigment of a pale brown tint, 
yellow by transmitted light, occurs on the sides, especially along the median lateral line. 
Twenty hours later, black spots, very minute in size, appear, intermingled with scat- 
tered yellow spots over the trunk and tail. The yolk, however, is devoid of pigment. 
PL XIX. fig. 5, shows the arrangement of the yellow pigment at the time of hatching. 
In examples at an advanced stage, e.g., twelve or fourteen days after hatching, a 
remarkable distribution of these spots is exhibited (PL XVI. fig. 1). The brownish 
yellow spots extend above the mid-brain (mb), around the eyes, along the mandibles, • 
and over the abdominal region ; but are especially aggregated along the dorsum upon 
each side of the median fin. The peculiar patches of radiate or stellate yellow spots which 
appear midway along the embryonic caudal fin-membranes, dorsally and ventrally, will be 
described in a subsequent page (see Median Fins). Eadiate black spots also occur 
amongst the yellow pigment. 


The dab (Pleuronectes limanda) has a distribution of pigment similar to that in the 
flounder, though the yellow spots seem to take a more distinctive linear disposition, two 
lines running along each side of the embryo, the upper line marking the dorso-lateral limits 
of the neurochord (PI. V. fig. 11). This distribution is well seen when the embryo is viewed 
from above. Pigment (yellow) appears when about thirty protovertebrse are outlined 
(i.e., about the seventh day after fertilisation). On the fourteenth day (two days after 
emerging) the pigment-spots around the margin of the eyes and the otocysts coalesce to 
form larger patches, irregular in form. A few days later, the upper lobe of the caudal 
membrane is diversified by the development of an undulating line of yellow pigment, or 
rather of a linear series of crescentic patches. Other spots occur thickly in the anal region, 
but the yellow pigment of the trunk is confined for the most part to two lines, as above 
described (PI. XVI. fig. 6). In a more advanced embryo, thirteen days after extrusion, 
the crescentic series of patches in the caudal fin is still more boldly marked, while two or 
three irregular touches appear on its lower lobe. The stellate pigment-spots are now 
meagre, occurring, as in the earlier stage just described, over the eyes, along the ventral 
region, over the greatly diminished yolk-sac, and very sparsely on the tail. The eyes 
have become darker, by increase of their black choroidal pigment, and about this time 
they show a striking green lustre in oblique light (PL XVI. fig. 3). 

In the plaice (PI. V. fig. 6) black pigment-spots, mingled with finely stellate 
bright canary-yellow corpuscles, develop, though comparatively late, and when the embryo 
is freed it does not show the marked pigmentation of the cod or like forms. On the 
third or fourth day after emerging yellow pigment appears as very minute amorphous 
spots. In PI. XVI. fig. 5, the peculiar distribution of the two tints is seen. The head 
and trunk present very minute, scattered spots. The ventral margin of the alimentary 
tract shows stellate black spots ; while the upper and lower contours of the caudal region 
have bold lines of stellate spots, which extend to the caudal fin-membrane, though con- 
fined to the lower lobe, and here the spots are simple and very minute. The yellow 
pigment appears only as a narrow area towards the end of the tail, viz., the upper 
margin of the posterior half of the caudal trunk. At the root of the tail a dense patch 
of black spots occurs, extending obliquely just above the urinary vesicle. 

Pigment appears in the gurnard at a slightly later stage than in the foregoing forms. 
It consists of very pale yellow spots, which have a delicate sea-green tinge in certain 
lights. They are sparsely scattered over the trunk proper, but form a rude line along 
the dorsum, and an undulating line along the sides and around the eyes. Three or four 
days later minute black spots occur, and both colours are sparsely distributed over the 
yolk-sac, and around the large oil-globule. A more advanced embryo is seen in PI. XVI. 
fig. 8, at which stage irregular patches of yellow and black pigment exist upon the 
dorsal and ventral portions of the caudal membrane. The spots send out branched 
ramifying processes, and the pectoral fin exhibits distally a radial yellow and black 
coloration. The eyes, however, are very slightly tinted with minute black spots. In 
still later larval and post-larval stages the pigment diminishes, and only occurs very 


sparsely in linear areas along the summit of the head, the opercular region, and on the 
snout. In form the spots are amorphous or rudely stellate. Along the huge pectoral fins 
and the ventrals similar minute corpuscles are developed, mingled in the former pair of 
fins with yellow pigment-spots (PL XVII. fig. 5). 

Certain features in the development of the pigment are noticeable, such as the fact that 
in some Gadoids the spots are confined solely to the trunk (cod, haddock, and rockling), 
(vide PL XVII. figs. 1,2); while in the whiting (PL XVI. fig. 2), the sole and the ling 
(PL V. fig. 9), the covering of the yolk (y) becomes richly pigmented. This pigmenta- 
tion of the yolk-sac is a feature also in the gurnard (PL XVI. fig. 8), and in the latter and 
the ling coloration is preceded by the appearance of colourless corpuscles, which are 
scattered over the yolk-sac {vide PL XIX. fig. 9). Pale neutral tinted bodies, evidently 
protoplasmic, and of various angular shapes, are distributed over the yolk-surface. They 
send out pseudopodia, and become rudety stellate, In the ling this occurs on the fifth 
day after fertilisation — about the time that the blastopore closes ; and in the gurnard at 
a similar stage these protoplasmic particles with short processes also appear. These 
bodies are obviously only a cortical disposition of protoplasm — less delicate and complex 
than the elaborate network of protoplasmic threads which extends over the yolk-sac in 
the cod, haddock, flounder (PI. XIX. fig. 5), dab (PL V. fig. 3), and other forms. 

The pigment-spots which occur over the yolk-surface are beneath the cellular germ- 
layer. They develop, as Ryder has pointed out, in the non-cellular periblast ; and Cun- 
ningham, while noting this condition, viz., that " they are situated at the surface of the 
periblast," in Pleuronectes microcephalics and Scomber, states that in the latter species the 
pigment is confined to the deep surface of the oil-globule and the sides of the embryo. 

If the large multipolar corpuscles in the ling and gurnard be merely the nodes or 
thickened points of intersection for the protoplasmic threads crossing over the whole yolk- 
surface, it is remarkable that these points of intersection should not develop pigment in 
the cod and dab, whereas they apparently become the pigment-spots of the yolk-sac in 
the ling and gurnard. The actual transformation of the colourless corpuscles into 
pigment-spots was not observed, but it is very probable.* 

The pigment-spots of the embryonic trunk often form distinct papilliform projections, 
the growth of the corpuscle pushing the epiblast out, and forming a small mound at that 
point. If the development of a pigment-spot be followed in the ling or gurnard (vide PL 
V. fig. 2), we see a rounded or irregular particle of clear protoplasm superficially placed 
upon the yolk-surface, which shows amoeboid movements, and sends out blunt processes 
(PL I. figs. 8a, 86). These processes become bifurcate, and assume a more or less elabo- 
rate ramose disposition — a stellate corpuscle being the result (PL V. fig. 2b). In the 

* In Gastrosteus Kupffer speaks of the appearance on the yolk-surface of small nuclear bodies, from which he says 
not only pigment, but blood-corpuscles are formed. These nuclei, probably the nuclear bodies already referred to in 
the allied marine species (p. 55), which become radiate in form, develop pigment-particles, the others keep their 
original shape until they are set in motion by the establishment of a blood-circulation (No. 88, 1868). In Gastrosteus 
spinnr.hia, the yolk-cortex, even before the blastopore closes, presents a striking appearance on account of the large 
translucent nuclei which are scattered all over it. These nuclei often show many nucleoli (vide No. 124, p. 493), and 
in the freshwater species, G. aculeatus, a reticulation is also present, but this has not been observed in G. spinachia. 


centre a nuclear portion (n) can be made out, and this usually remains clear and un- 
changed, while around it very minute particles of black pigment (pt) develop. These 
particles increase so rapidly that the bases of the pseudopodia become much darkened, 
and a centrifugal transference commences, the minute particles flowing along the ramify- 
ing arms, until a pale steel-tinted stelliform body becomes distinctly outlined. The tint 
grows in intensity, and finally shows the dense black colour characteristic of the completely 
developed corpuscle. In many cases by their extension these black corpuscles intermingle 
so as to interlace their arms in a complex manner, and even coalesce, as was noticed by 
Lereboullet, who also observed the persistence of the central pale nucleus in each 
corpuscle (No. 93, p. 579). 

The variations in the disposition of the pigment in different forms is noteworthy, and 
its diagnostic utility has been already mentioned. The time at which pigment appears is 
also remarkable. Lereboullet found in Perca that it develops earlier and more abund- 
antly than in Esox, though in both forms it overspreads the yolk-sac (No. 93, 
pp. 579-586, 610). It is very precociously developed in the flounder, and comparatively 
late in the whiting. 

During the later larval stages the epidermis becomes very irregular — rounded pro- 
tuberances appearing especially over the cranial and facial regions (PL IX. fig. 3 ; PL 
XVII. fig. 4). Many of these are sensory enlargements, and described elsewhere, but 
enlarged mucous cells develop, especially in the region of the snout. These open 
superficially, and doubtless are protective in function — bathing the young embryo exter- 
nally with a gelatinous secretion. The contents of these large mucous cells stain very 
deeply, and are especially noticeable in sections of the plaice, though in Cyclopterus and 
others they also form a noteworthy feature. 

No cilia are apparently developed upon the embryonic integument, nor do fine 
immovable hairs occur as in Petromyzon and its young stage — Ammoccetes. The serial 
sensory papillae (PI. VI. figs. 8, 8a) send out fine filiform processes (pip), but they are 
local, and probably pushed through from the neurodermis below. The development of 
scales as protrusions from the corium which burst through the epiblastic integument, as 
well as the formation of iridescent plates in the stratum Malpighii, belong to a late post- 
larval stage. In some young forms, it is true, a brilliant iridescent appearance is seen in 
the abdominal region ; but this is occasionally due to the enlarged swim-bladder, the fishes 
in certain cases remaining translucent, and almost colourless in the post-larval stages, when 
all the more important structural features of the adult are assumed. In such forms, again, 
as the post-larval Anarrhichas, the whole abdomen is iridescent. 

Ova and Generative Organs. — As soon as the segmental ducts have reached their 
final position on each side of the dorsal aorta, a strand of peritoneal (splanchnopleuric) 
cells passes below them. They thus become grouped on the inner side close to the 
mesentery (PL VII. fig. 1). These cells become aggregated, and produce an irregular 
contour especially in the posterior region — where the alimentary canal is more distant 
from the notochord, and the median mesenteric membrane is better developed. They 

VOL. XXXV. PART TIL (NO. 19). 6 I 


form, in fact, the germinal epithelium, but a definite germinal ridge cannot be made out. 
Indeed, in the haddock, it is not until the second or third week after extrusion that this 
germinal portion becomes distinctively marked (PL XL fig. 14). Some of these cells 
(po) are seen to enlarge and protrude from the surface of the mesentery (msn) into the 
abdominal cavity as large primitive ova, and they occur, almost solely, slightly anterior 
to the urinary vesicle, especially above the region of the small intestine. In short, their 
appearance and distribution precisely accords with Balfour's description of the early 
Elasmobranch ovum (No. 15, vol. xi. p. 161). The ova are most closely grouped on 
the roof of the abdominal cavity, and especially in the median niches formed by the 
projection of the suspensory septum or mesentery (msn). They are also grouped upon 
the mesentery, and some develop upon or have migrated to the peritoneal envelope of the 
intestine itself (hg). They are very irregularly distributed, and show great variation in 
size ; large spherical ova projecting from a mass of small undeveloped cells, and all 
loosely held together by the delicate connective tissue of the peritoneum. The ova 
appear to be like the cells adjacent, and differ only in their larger size and more active 
development. Each consists of a mass of minute nucleated spheres enclosed in a thin 
membrane ; but are quite unlike the primitive ova of Elasmobranchs, as described by 
Balfour (No. 13, p. 164), for these latter are uninucleate, one or two nucleoli, stain- 
ing deeply, occurring in the nucleus, which is large, and surrounded by a granular 
protoplasmic matrix. Along each side of this region of the abdomen, external to the 
abdominal cavity, a mass of cells may occur, not unlike, but less in dimensions than, the 
primitive ova described above. The lateral niche in which they are aggregated is defined 
by richly pigmented peritoneum, and this pair of lateral sacs strongly suggests the 
ovaries of the adult. The largest ova are those, however, which are free, and project 
boldly from the mesentery and roof of the abdomen. Balfour speaks of a thickened 
germinal epithelium in the Teleosteans, into which the adjacent stroma sends ingrowths — 
the cells of the epithelial layer increasing by the growth of the clear protoplasmic 
contents. But this does not correspond with the condition seen in the young haddock, 
each ovum being a more or less perfect sphere, and enclosing numerous minute 
nucleated bodies. Later stages were not observed, and it was not made out whether the 
lateral peritoneal sacs finally became the ovaries with their continuous genital ducts, or 
whether an epithelial layer grew over the freely suspended primitive ova, and enclosed 
them in an ovarian sac, depending from the abdominal roof. 


IX. The Fins. 

Median Unpaired Fins. — The development of a median epidermal crest (ef, PI. V. 
fig. 11 ; PI. XIII. fig. 3 ; PI. XIX. fig. 10), extending along the median dorsal line from 
the cephalic region round the end of the tail, and along a portion of the under surface of 
the caudal trunk, is an early and noticeable feature in the embryos of Teleostean fishes, 
with probably few exceptions (e.g., Hippocampus). Soon after the tail is detached from 
the yolk-surface, within a day or two after the closure of the blastopore, a minute fold of 
epiblast projects as a ridge along the whole course just indicated. It grows in vertical 
breadth, being pushed out in the form of an epiblastic fold, and shortly before the extrusion 
of the embryo is quite a broad membrane, especially well developed in the hind trunk and 
caudal region. On account of its superficial extent — while the embryo is within the egg 
— it is creased and much folded about the body ; but on the embryo issuing from the 
ovum the membrane rapidly straightens out and becomes erect. It apparently continues 
to grow after extrusion, a newly hatched embryo having a much less extensive median 
membrane than one a few days old (compare PI. XIX. fig. 5 ; PL XIII. fig. 6 ; PI. XVI. 
fig. 1). The extent covered by this fin (ef) varies in different species, thus in the young 
of Trigla gumardus (PL XII. fig. 1) it never extends quite so far forward as in the forms, 
e.g., Gadus ceglefinus (PL XIV. fig. 1), G. morrhua, G. rnerlangus (PL XVI. fig. 2), and 
Motella (PL XVII. fig. 2) ; its wider portion in fact reaching only to the otocystic region, 
in front of which its height gradually diminishes, and the fin disappears above the 
occipital region (PL XVI. fig. 8). In such examples as the Gadoids just mentioned, it is 
broad and prominent as far forward as the mid-brain, in which region it gradually 
slopes to a mere ridge. The thinness and transparency of this structure is remarkable. 
It is so delicate that as the fish progresses through the water it is flexed and waved 
about with every movement, and on removal from the water the fin collapses at once, 
and lies like a film on the body. Slight contact with a hard substance immediately 
injures it, and while in healthy larvae it stands out erect and even, and is per- 
fectly translucent, it appears crumpled and in many parts opaque when the fish is 
in a sickly or dying condition, ultimately dissipating or breaking up into needle-like 

In certain forms, e.g., Gadus rnerlangus (PL XVI. fig. 2), Molva vulgaris (PL XVII. 
fig. 9), and Solea vulgaris (PL XVII. fig. 13). the pigment, which extends not only over 
the body, but over the yolk-sac, appears also upon the embryonic fin (ef) ; whereas in 
Gadus morrhua, G. ceglefinus, &c, no such pigment-corpuscles occur save on the trunk 
of the fish — the yolk-sac as well as the membrane being destitute of them. It was men- 
tioned previously that in Pleuronides limanda (PL XVI. figs. 3, 6) and Trigla gumardus 
(PL XVI. fig. 8) the fin shows during the later larval stages remarkable coloration 
— in the former species crescentic particles of yellow pigment appearing in regular series 
along the membrane above and below the caudal trunk during the second week after 


hatching, while in the gurnard a third-day embryo shows irregular patches of yellow pig- 
ment, with which black spots are also mingled (PL XVI. fig. 8). The coloration in other 
species will be noticed on a following page. 

In transverse section this fin-membrane (ef) consists merely of a simple median fold 
of the double-layered epiblast — the outer flattened corneous layer, and the inner sensory 
layer, which proceeds into the narrow fissure separating the two lamellae of the fin 
(PI. VII. figs. 3, 6). This fissure enlarges close to the trunk, and is continuous with a 
spacious subepidermal chamber which extends all round the latter, and is well seen in 
late larval stages in section (PL VII. fig. 6) and surface view (PL XVI. figs. 1, 3). A jelly- 
like lymph fills up this cavity, which, as already pointed out, becomes extraordinarily 
enlarged in the cephalic region. All along the trunk such a space exists in a modified 
degree, and delicate nerve-strands pass across it from the spinal cord to the sensory 
papillae in the skin. Along the tail the interspace is narrowest (ss, PL XL figs. 15, 17), 
but on the ventral side, as the root of the tail is approached, it enlarges and forms a 
spacious fissure in the anal region (ss, PL XL fig. 14). It is in this chamber, limited 
on each side by the epiblastic fin-fold, that the rectum (hg) pushes its way, and before 
the anus is formed sends out a strand of loose cells, extending from the base of the 
urinary vesicle to a point midway down the expanse of the fin-membrane. 

The hind gut, as already indicated, ends blindly, and does so for a period varying 
very much according to the species. The anal column of cells, before and after a 
lumen is formed, passes down the centre of the fissure (ss), and is apparently held in 
place by the tenacious plasma (x, PL VII. figs. 12, 13), in which granules subsequently 
appear, and forms a matrix surrounding this part of the intestinal tract. As formerly 
mentioned, the anus does not extend to the ventral margin of the fin, but opens at the 
side about midway (a, PL VII. figs. 14, 15). In this continuous embryonic fold the 
permanent unpaired fins of the adult fish are formed — arising, as Balfour said, by local 
hypertrophy (No. 11, p. 78), though no less by atrophy of the parts between the 
ultimate fins. Lereboullet refers to this atrophy in Perca, when he says the margin 
becomes indented where the three vertical fins in that species will finally remain 
(No. 93, p. 634). These local indentations mark the atrophy of parts of the embryonic 
membrane, which finally disappear, leaving the prominent and strengthened remnants 
of the once continuous fin to form the permanent unpaired fins. Before this atrophy 
of the transient portions and the hypertrophy of the permanents parts, the sites of the 
ultimate fins often appear to be indicated by remarkable aggregations of pigment. Thus, 
in the advanced embryo of Pleuronectes jiesus, a striking development of pigment- 
corpuscles takes place in the dorsal and ventral portions of the embryonic fin. Scattered 
pigment occurs along its whole extent behind the pectoral region, though it is sparse ; 
but certain parts in an early stage are distinguished by more abundant coloration, and 
in the thirteenth-day flounder, referred to, a patch of brownish-yellow pigment-spots, 
arranged in a radiate manner, is seen with black spots intermingled (PL XVI. fig. 1), as 
also in the undetermined Pleuronectid figured on PL XVIII. fig. 1, and in Agonus on the 


same plate, fig. 11. A similar dorsal and ventral arrangement of caudal pigment-spots 
occurs in the advanced embryo of the ling (PI. XVII. fig. 10), black pigment-spots 
diverging upward and downward from the caudal trunk in a characteristic manner. In 
this way the sites, so to speak, of the future median fins are indicated by radiate 
coloration before the continuity of the embryonic membrane (ef) is to any appreciable 
extent destroyed. Later, however, the developing fin-rays (embryonic) are more clearly 
indicated by granular striations which pass across the membrane (vide PI. XTII. 
figs. 2, 6a ; PI. XV. figs. 4, 5), still very thin and transparent (though a fine reticulation 
of a superficial character often appears in it), no mesoblast having as yet insinuated 
itself into the interlamellar fissure, as shown in a section of the haddock on the 
third day after hatching (PI. VII. figs. 3, 4) ; or even so late as the seventeenth day 
(PL XI. fig. 14). Lereboullet noticed similar indications in the still persisting 
membrane of the embryo of Perca when twelve to fifteen days old. He describes along 
its whole length small irregular transparent structures like oil-tracts, and he found that 
they accumulate where the permanent fins will be developed (No. 93, p. 640). These 
are either the homologues of the pigment-corpuscles mentioned above, or aggregations of 
the external reticulation. Later, he says, he noticed these disappear in Leuciscus eury- 
ophthalmus as if by absorption, and striations inclined in a backward direction take their 
place. They form successive pairs, the rudimentary rays, in fact, of the unpaired fins, 
which he remarks are double at the time of origin (p. 640). Eyder speaks of the 
mesoblast as entering the fold at an early stage (No. 114, p. 517),* but this does not 
apply to many forms, for a section through an advanced embryo of the haddock, as just 
mentioned (PL XI. fig. 14), still shows a mere epiblastic fold (ep) little altered from its 
primitive condition. While the membrane still remains thin and translucent, ray-like 
thickenings are frequent — apparently aggregations of a horny or chitinous nature, usually 
regarded as epiblastic thickenings, which develop, as Lereboullet observed, centri- 
petally, and grow towards the trunk (No. 93, p. 637). He describes them as transparent 
strips, distant from, but directed towards the body, and appearing simultaneously in the 
three parts which subsequently form the three vertical fins in Leuciscus euryophthalmus. 
These rays Lereboullet describes as formed by a " condensation of a plastic material 
without any grouping of cells," and he regards them as connected with the vertebral 
column below from which they are separated, subsequently, by the interspinous bones 
(p. 630). In reality, however, the early rays are merely dermal thickenings, and appear at 
first as narrow granular tracts indefinite in outline, and extending dorsally and ventrally, 
and therefore unconnected with the axial skeleton below. Lereboullet's view applies 
to the dense permanent rays which develop in the post-larval stages, for these rods 
are paired, and arise under the epiblast — beneath the pigment, which appears in the 
Malpighian layer of the ectoderm, and are most probably aggregations of mesoblastic cells 
which grow up into the median fin-fold from the axial (skeletal) mesoblast below. In 

* Ryder now holds that even the embryonic fin-rays are mesoblastic {Rep. U.S. Comm. Fish and Fisheries, 1884). 
As fast as they appear, they are preceded or accompanied by outgrowths of mesoblastic cells. 


the embryos of species with pelagic ova, e.g., Gadus morrhua, G. ceglefinus, G. mer- 
langus, Molva vulgaris, Trigla gurnardus, and the Pleuronectidse, such median fin- 
rays do not appear even in the late larval condition (ef, PI. X. figs. 1,4; PL XVII. 
figs. 2, 10, 12) ; and Eyder instances other examples, some having demersal ova, e.g., 
Alosa, Pomolobus, Cybium, Parehippus, and Idus, in which this is so (No. 141, 
p. 518), the original transparent membranous condition of the embryonic fin persisting 
to a late stage. Eyder adds that in Gambusia and certain Lophobranchs no embryonic 
fin-fold is formed at all — the single dorsal fin arising later as a local dermal excrescence 
with a core of intruding mesoblast (No. 141, p. 518). In some Cyprinoids (Idus and 
Carassius), which also possess a single dorsal only, the continuous embryonic membrane 
nevertheless appears. PL XVII. fig. 5, represents a young gurnard in which the two 
dorsals and the single anal fin (of) are indicated; but the former are still continuous 
with the tail-membrane (cf), while a remnant passes forward to the anal fin. The stage 
figured is post-larval, and in PL XVII. fig. 7, the fins have really reached the adult 
condition, and are completely differentiated, all trace of the continuous embryonic mem- 
brane having disappeared. In PL XV. fig. 6, representing Cyclopterus lumpus, these 
intermediate connections are still discernible, though the two dorsals (df) and the 
anal are almost wholly separated. These unpaired fins have become conspicuous 
by hypertrophy at three points, and by the atrophy of the membrane in front and 
behind (see also fig. 5). The ventral median fin is broken up into two by the anus (a), 
which, e.g. in Gastrosteus, has pushed its way down and terminates at the apex of an angular 
bay marking off a pre-anal from a true anal fin (PL XV. fig. 5). Lereboullet describes such 
a bay in the newly emerged embryo of Perca, while the body is still encircled by the con- 
tinuous fin, " the lower edge," he says, " exhibits an indentation where the anus will appear " 
(No. 93, p. 616). This condition differs very much, it is unnecessary to point out, from 
that in the newly hatched embryos of the species here described. A post-larval flounder, 
5 "8 mm. in length, which is perfectly translucent and colourless, but has lost almost every 
embryonic trace, still retains a membranous vestige connecting the dorsal above and the 
anal below with the caudal fin. The three fins thus connected have otherwise attained 
all the characters seen in the adult. 

The unpaired fins in Teleosteans, therefore, do not arise as two apposed, independent 
epiblastic plates, but as a median fold or crest. 

Prof. Humphry first broached the idea, from an examination of the adult anal fin, 
that it might be double in its origin, i.e., a union of two lateral fins ; and he suggested 
that the other median fins might have thus originated, and that the paired and unpaired 
fins were alike double primitively (No. 72), a view supported by the fact that the dorsal 
fins, in addition to their (spinal) motor nerves, are supplied by a pair of sensory nerves 
which branch off" from the trigeminal soon after it emerges from the roof of the skull. 
The study of their development, however, would seem to yield an opposite conclusion — 
the median fins are single at their origin * and their bilateral structure — muscular and 

* Lericboullet's statement that the dorsal fin is double at its origin is likewise misleading (No. 93, p. 630). 


skeletal — is subsequently assumed, and it must be added the paired sensory (nerve) con- 
nection, above mentioned, is probably also secondary, for in Selachians and Dipnoans no 
trace of it is seen. It is remarkable that in some forms, e.g. , Apeltes, vascular loops are 
formed in the median fin-folds at the time of hatching, whereas in none of the species 
specially referred to in this paper is this the case — the membranes remaining transparent 
and non-vascular for some time after extrusion. 

The Caudal Fin. — It is plain from the foregoing observations that the tail of the 
embryo is not by any means distinctly marked off from the trunk. The tail is indeed 
the tapered portion of the trunk, which gradually diminishes, ending posteriorly as a thin 
rod (the notochord with the muscle-plates on each side), and the neurochord above (cf. PI. 
XIII. figs. 1, 2, 4, 6a, 7). A strand of connective tissue passes along beneath the rod, 
and in this tissue the haemal trunks of the tail are by and by formed (vs., PL VII. fig. 6a ; 
PL XL figs. 15, 17). The whole is encircled by the embryonic fin-membrane (ef) 
which passes along the median dorsal, terminal, and ventral line, so that the tail is at 
this early stage of the typical protocercal type, showing no division into lobes. In the 
early ling, about the time that the eyes are fairly complete, two peculiar folds are sent off 
below the muscle-plates in the caudal region. While within the ovum the caudal trunk 
lies for some time as a flattened process upon the yolk, its greatest breadth being at right 
angles to the caudal plane of symmetry, and when first it buds out from the trunk it 
is in a state of torsion, the developing fin-membrane being folded in a complicated manner 
at the root of the tail, and passing as a horizontal ridge round its termination (PL II. 
fig. 11). This state of torsion, which is very marked in the earliest condition of the tail, 
does not continue, and shortly before hatching the enlargement of the perivitelline space 
not only gives the caudal trunk more freedom, but even permits active movements on 
the part of the embryo. 

Usually, as pointed out above, the trunk terminates in a more or less accuminate 
process (PL XIII. figs. 1, 2, 4, 6a, 7); but in PL XV. fig. 4, a remarkable terminal 
enlargement is seen, the neurochord swelling to form a lobe, while the notochord ends in 
an irregular bulbous structure. In the figure just referred to (PL XV. fig. 4) the tail- 
fin proper is marked by a radial structure (embryonic fin-rays), apparently a mere 
dermal thickening, such as we see in a late stage of Pleuronectes limanda (PL XVI. fig. 
3). In PL XIII. figs. 6, 6a, the embryonic membrane is diminished between the ter- 
minal caudal and anterior portions, and a mass of granules is forming around the end of 
the notochord, which assume a radial disposition. These diverging granular tracts are 
better defined, and form, in fact, rays in the dorsal and ventral lobes of the membranous 
fin of the same embryo (PL XIII. fig. 7). The formation of fin-rays, without the 
intervention of special cellular prolongations from the vertebral arches, was observed by 
Lereboullet, who speaks of them as produced probably by " the deposit of a cartilaginous 
cytoblastema " (No. 93, p. 634). The appearance of these rays does not suggest a 
cartilaginous character, the fine granular tracts (PL XV. fig. 5), as they become defined- 
form clear translucent rods (PL XIX. figs. 2-4), not unlike the " spicular substance," 


which appears in certain parts of the axial and appendicular skeleton (e.g., vertebral 
bodies and pectoral arch). Most probably they are of a resistent horny (?) nature, and 
they are developed at first in the distal or mid-part of the fin-membrane, approaching, 
as before pointed out, the trunk by the growth of the proximal end of each ray, " their 
development being in conformity with M. Serres' law of centripetal formation " (No. 93, 
p. 634; also vide Serres' Principes d'organogenie, Paris, 1842, p. 212). As the rays thus 
develop, the aboral end of the cellular notochord (nc) curves upward (PL XVIII. fig. 3), the 
upper lobe (opisthure of Ryder) diminishes, while a new and larger lobe expands on the 
ventral side of the chorda. A notch, however, separates this new growth from the lower 
lobe of the primary protocercal tail (PL XIX. fig. 4).* Agassiz describes this development 
of the secondary caudal membrane as an atrophy of the upper lobe, and a rapid 
development of the low r er lobe which becomes bifid. The lower lobe does not really 
become bifid, but a new lower or rather anterior ventral lobe grows out, and by its 
rapid development leaves a notch separating it from the primary lower lobe. The two 
original lobes of the protocercal tail are gradually pushed further up and almost entirely 
disappear, the tail of the adult being for the most part a wholly new growth on the 
ventral side of the notochord, and slightly anterior to its termination (compare figs. 3 and 
5, PL XVIII.). The stages of this atrophy of the primary fin-lobes and the growth of the 
secondary tail-fin, mainly as a new product, can be seen by comparing PL XVII. fig. 3, 
which shows the original protocercal outline, with fig. 6 on PL XV., in wdiich the 
secondary tail-fin is formed as a large ventral lobe supplanting the primary tail. 
In fig. 5, PL XVII. , the new tail-fin has completely taken the place of the primary 
membrane. PL XVIII., figs. 3, 4, 5, and 7, show these stages well. The embryonic tail with 
its dermal rays is transitory, and the permanent tail with its hypural elements (PL XV. 
fig. 3) belongs to a stage which is post-larval. Lereboullet says the materials out of which 
these later skeletal elements are developed are furnished by a rich caudal plexus of blood- 
vessels. This complex vascular development, he says, "precedes and announces the 
formation of the tail," and it consists of a system of elongated loops in the pike, perch, 
trout, and roach (No. 95, p. 26). No such subnotochordal terminal plexus is formed in the 
Gadoid and other forms studied at St Andrews. Thus the gurnard, even at so advanced 
a larval stage as PL XVII. fig. 5, shows no such network; yet the hypural plate is well 
developed and the fin rays fully defined.f 

The Paired Fins. — When the embryo is first outlined in the blastoderm, an alar 
expansion stretches away on each side of the trunk of the young fish. This expansion 
consists of epiblast and hypoblast resting upon the stratum of periblast below. No 

* Ryder (" Evolut. of Fins of Fishes," Report of Com. Fish and Fisheries for 1884-1886) states that there is evidence 
of the degeneration of the caudal region, as in Chimcera and Stylephorus there is a permanent archicercal opisthure, a large 
temporary one in Lepidosteus ; and, moreover, there is the evidence of the concrescence of the hypural pieces ; the 
ventrally diplacanthous and even triplacanthous caudal vertebra, or their coalesced representative, the urostyle ; the 
existence of hypaxial opisthural elements ; the abortion of the epaxial spines of the caudal vertebra3 ; and finally, the 
abortion or extreme modification of the last muscular somites of the caudal region. Ryder [op. cit., from an examina- 
tion of ih I) lmlds that the hypurals are partly haemal and partly interspinous. 

+ See Lotz on "Tail of Salmon," &c, Zeitschr. f. wiss. Zool., 1864, p. 260. 


mesoblast apparently extends into it (al, PI. III. figs. 11-13; PI. IV. figs. 4, 10), 
though this layer is ill-defined laterally at this stage. A pair of lateral horizontal alee 
(al), indeed, stretch along the whole trunk — from the pectoral to the post-mesenteric 
region. It is in reality the elongated and narrowed blastodermic scutum (PL XXVIII. 
fig. 5), and extends in front and behind the two points mentioned, though it is there 
thinner and hardly distinguishable. In PL III. fig. 19, such a pair of lateral horizontal 
fin-expansions are present extending from the trunk-region proper, and their limits 
are very definite when viewed from above. Just as in the case of the median vertical 
fins, certain areas in these horizontal alee become defined, as special fin-regions by a 
visible thickening, apparently from the folding under of the epiblast. Thus two flattened 
oval pads consisting of a double epiblastic fold like the double median fin-fold, are 
disengaged from the rest of the alar expanse. Before and behind this pair of pads the 
lateral membrane thins away and atrophies, while the special portions continue to increase 
in density as a pair of pectoral limbs (pf, PL V. figs. 6, 9; PL XIV. fig. 1). Lereboullet 
apparently did not notice that the pectoral fins emerge from the lengthy lateral mem- 
brane or alar expanse on each side, and speaks of a gradual accumulation of cells from 
the inferior lateral portions of the trunk as a pair of tubercular processes protruding- 
some distance behind the ears. In Perca he found that these fin-pads became detached 
on the seventh day (No. 95, p. 10 ; No. 93, p. 583). The increasing density of the fin- 
pads is due to the entrance of mesoblast into the interstice, separating the upper from 
the lower epiblastic lamella. This mesoblast spreads out radially, but does not reach 
quite to the distal margin, and the peripheral portion remains more transparent, though 
the epiblastic cells which solely constitute it become columnar, and form a thickened 
ridge from which the fin-rays doubtless subsequently develop centripetaily.* Such a 
mode of development as that above sketched has theoretical bearings of considerable 
interest. These were briefly treated in a former note (vide No. 124a, p. 697), and need 
not be discussed in this place further than to point out that the fin develops as a 
horizontal ridge, in accordance with Balfour's theory of a primitive horizontal lateral 
fin, and that it is independent of and prior to the formation of a girdle-rudiment. Prof. 
Cleland, in a paper on the Limbs of Vertebrates (No. 40), emphasised this latter point, 
and further showed that a limb involves two distinct elements — a radiation (i.e., an 
appendix) and an arch, which is not a radiation, but a cincture, which always circles more 
or less round the body, and may be complete above or below. Prof. Cleland further 
stated that neither appendage nor limb-arch is the property of one particular segment, — 
their position being variable and their nervous supply multisegmental, — points which are 

* Kingsley and Conn, in the cunner (Tautogolabrus adspersus, Gill), and other authors in various forms, have 
recognised only the lateral fins when they were defined as tubercular pads. The observers named speak of these fins 
as only developed when the embryo is ready to emerge — the tail being free and the capsule loosely surrounding the 
fish (vide No. 78, fig. 51, pi. xvi.). No trace of a continuous lateral fold could be seen, the fins protruding as simple 
outgrowths (p. 210). The extension of the thickened epiblast and hypoblast laterally is, however, a feature common 
to all Teleostean embryos, and a portion of this becomes defined in all the forms studied at St Andrews, and out of 
this defined epiblastic fold the pectoral fins arise. 

VOL. XXXV. PART III. (NO. 19). 6 K 


supported most clearly by the development and early condition of these structures in 

From the primary horizontal position (pf, PI. V. figs. 4, 6, 9 ; PI. XIX. fig. 7), 
the fins change to a more vertical situation (pf, PL XIII. figs. 1, 6 ; PI. XVI. figs. 
6, 8), though still connected by a lengthy attachment to the side of the embryo. The 
mesoblast of the fin-plate may be traced to a mass of cells in which the Wolffian ducts 
lie, and out of which they are developed (PL VII. figs. 1. 2). If these ducts, as appears 
to be the case, arise as lateral ridges or diverticula of the somatopleure, then the meso- 
blastic cells of the fins must be pronounced somatopleuric. But no ridge of somatopleuric 
cells, comparable to the Wolffian ridge of higher forms, has been recognised in fishes, and 
we must regard this mesoblast as indifferent, and forming an "intermediate cell-mass" 
adjacent to the excretory system. The proximity of the Wolffian duct and the base of 
the pectoral fin is very noticeable (PL VII. fig. 7). The fins gradually become dis- 
connected from the blastodermic yolk-sac, and about the time that they are free a median 
stratum in their mesob]ast assumes a columnar character, and is seen as a transversely 
striated central bar in cross-section (x, PL VII. fig. 2). This plate (x) is gradually con- 
verted into cartilage, and extends from the base of the fin, where it is thickened almost to 
the distal border, at which it thins out and ceases (PL VII. figs. 1-3). Around this fan-like 
cartilaginous plate the adjacent mesoblast develops rapidly, especially near the proximal 
attachment to the trunk, so that a stout peduncle is formed (PL VII. figs. 1, 2). 
Viewed from above, in the living embryo, the fin appears as in PL VII. fig. 10, the outer 
and anterior margin presenting many protoplasmic processes, which seem to bind it to 
the epiblast over the yolk. The pigment-corpuscles, moreover, may be regularly disposed 
on the fin. Each fin, therefore, consists of a thickened stalk and an outspread distal 
expansion (pf, PL XII. fig. 6a), traversed from the base almost to the summit by a 
flattened plate of cartilage which is imbedded in a mass of indifferent mesoblastic cells, 
destined to become the muscles of the limb, and forming the main mass of the peduncle 
(PL VII. fig. 7). The basal part continues to become thicker, and later is disproportion- 
ately enlarged, while at the same time the more distal parts expand like a fan, and 
become thinner and more transparent, save where the delicate radial striations pass. 
The part towards the distal border in many forms quickly exhibits pigmentation, e.g., in 
T. gurnardus (PL XII. fig. 1), Molva vulgaris, Cottus, and Liparis, radially disposed 
yellow and black pigment-spots being intermingled in the distal parts of the fin in the 
first-named species (PL X. figs. 2, 3; PL XVI. fig. 8), or again, rich orange stripes in 
Liparis (PL XVI. fig. 7). 

During the third week after hatching the " rotation " of the fin has reached a stage 
at which its position is seen to be wholly altered, the original horizontal position (PL 
XII. fig. l) being now exchanged for an oblique vertical attachment (PL XIII. fig. 
1 ; PL XVI. figs. 3, 4, 7). The rotation continues until its basal attachment is almost 
perfectly dorso-ventral, and therefore at right angles to its primary position (PL X. figs. 
2, 3; PI. XV. fig. 2; PL XVIII. figs. 2, 10, 11). Meanwhile a pectoral bar appears 


on each side of the thoracic region, extending dorsally and ventrally, forming in fact two 
halves of the pectoral girdle as yet disjoined below. Kyder distinguishes, before the 
development of the cartilaginous girdle, an oblique pectoral fold (No. 141, p. 520), con- 
sisting of a band of mesoblast, out of which, he states, the girdle develops. There 
appears on each side, therefore, a clear yellowish rod, tapering at its upper and lower 
extremities, and curved like an f, — as in the gurnard on the eighteenth day (cl, PL X. 
figs. 2, 3 ; also PI. XIII. figs. 5, 6, 7). In PI. XL fig. 18, this bar is figured as removed 
from a larval Pleuronectid about three weeks old. The species was not determined. The 
small triangular element attached, though not unlike the post-temporal, is probably the 
coracoid bone. This secondary bar may be readily recognised by its form and position 
as the clavicular element (cl), and it develops in certain species, as in the gurnard, 
the Gadoids, and others, without being preceded by a bar of cartilage-cells, and in these 
forms the basal part of the fin-cartilage is greatly developed, as if preparatory to inclusion 
as a posterior part of the girdle. If the homogeneous, translucent, brittle rod, strongly 
suggestive of chitin, be the clavicle, then the elements behind, which become attached to 
it, must be the scapular and coracoidal portions of the permanent girdle. By the 
breaking-up of the basal portion of the cartilaginous fin-plate the system of basilar 
pieces is formed (PI. XVII. fig. 5). Kingsley and Conn speak of this proximal car- 
tilaginous thickening as parallel to the axis of the trunk, and as preceding the distal 
rays. "This basal skeleton," they say (No. 78, p. 210), " instead of appearing as a pair 
of rods as described by Ryder, was rather a broad plate with a central opening, as if 
his rods had united at their extremities." The same feature was also seen in Lophius. 
There is much obscurity in regard to the development of the ultimate elements of the 
paired fins, and their relation to the axial girdles. The details of this further develop- 
ment, with the theoretical considerations involved in their interpretation, have been 
dealt with by one of us in a special paper.* 

Ventral Fins. — The development of the ventrals will be alluded to when describing 
the post-larval stages (vide PL IX. figs. 2, 3 ; and PL XVIII. fig. 3). They are late in 
making their appearance in the pelagic forms. 

X. Methods and Technique. 

I. Methods. — The ova and embryos are treated according to the usual methods of killing, 
fixing, staining, and cutting. Notwithstanding the large number of methods recommended by 
various embryologists, the ova and early embryos of Teleosteans may still be counted amongst the 
most difficult objects subjected to the microtomist's processes. The recommendations of various 
investigators are most conflicting, and a perfectly efficient and reliable killing, staining, and 
imbedding process continues to be a desideratum. Whitman, after trial of the usual hardening 
agents, " failed to find any completely satisfactory method of preserving the vitellus ; even the 
germinal disc cannot well be preserved by any of the ordinary hardening fluids " (No. 159a, p. 152), 
and this agrees with the common experience of investigators. 

* E. E. Prince " On the Development and Morphology of the Limbs of the Teleosts," Elizabeth Thompson 
Fund, U.S.A. 


The plan followed by Kupffer (His and Braune's Archiv Anat. Ahth., 1882), Henneguy (Bull 
de la Soc. Philom., Paris, 1879, pp. 75-77), and others involve too many processes to be adopted when 
the species studied are numerous, and the quantity of material is large. 

Various circumstances conduce to render Teleostean eggs difficult objects for treatment — not 
only on account of their small size, pelagic ova being rarely more than a millimetre in diameter — 
but the tough nature of the capsule and fluidity of the contents render the removal of the former a 
most delicate and hazardous task. If hardened before the capsule is removed, shrinking results ; 
and if the capsule be removed before hardening, the egg is more or less disorganised, unless the 
operator be very fortunate. The best sections are those gained by leaving the egg almost intact, 
and by hardening, staining, and imbedding in toto, but this plan is beset by many dangers. On 
removing the egg from the sea-water, and reference is made here to marine ova solely, the capsule 
is carefully pierced in order to facilitate the admission of the various media into which it is to be 
transferred. Save for this puncture, the egg is left entire, and thus it is passed through all the 
processes of killing, hardening, staining, clearing, and imbedding. 

The paraffin method proved to be the only practical one, other methods, such as imbedding 
in pith, which might serve for large eggs, such as those of the Salmonidae, were unsuitable 
for eggs so small and frail as those of the Gadoids, Pleuronectidce, &c* Various forms of the 
microtome were used in preparing the extensive series of sections of the various Teleosteans 
considered in these pages — the rocking microtome of the Cambridge Scientific Instrument Company 
being found very useful. The large Caldwell microtome, used in the classes of zoology at the 
United College, and kindly lent by the authorities of the University of St Andrews, was of great 
service ; while the Jung (Thoma's) microtome was found to be well adapted for older stages of the 
embryos, and for adult ovaries — a series of sections being cut by Dr Scharff. The sweeping 
motion of the last-named instrument proved very efficient in cutting through the more mature 
skeletal and other tissues of young fishes, to which task the fixed razor of the English microtomes 
proved unequal — refusing, in fact, to pass through the firm connective and cartilaginous elements. 

II. Killing, Fixing, and Hardening — Corrosive Sublimate. — The saturated solution is one of 
the most efficient killing and fixing fluids available in the laboratory, and it kills, fixes, and hardens 
so rapidly that Teleostean ova require to be left in it for a very short time. As soon as the 
penetration of the fluid is complete, they are removed and washed in dilute alcohol, rather than in 
distilled water. Washing must be well done, in order to prevent subsequent deposition of crystals 
in the tissues. The desirability of staining, clearing, and cutting after treatment with this fluid is 
too well known to require any explanation — the best preparations being found to be those in 
which, after killing and fixing, the subsequent operations are immediately proceeded with. A mix- 
ture of two parts corrosive sublimate and one part acetic acid was found to be most serviceable. It 
is a powerful killing and fixing fluid, and produces the best results. For killing, two or three 
minutes usually suffice, and washing is then done in very weak alcohol — the alcohol being 
frequently changed until the killing medium is wholly extracted, and graduated alcohols follow, 
viz., 30, 40, 50, and 60 per cent. 

Picro-sulpJmric Acid (Kleinenberg). This useful killing and fixing fluid does not produce 
the best results, since it frequently causes the blastomeres in early stages to expand and burst the 
capsule, thus entirely disorganising the embryonic structures. Whitman experienced the same 
results (op. cit, p. 152), but occasionally this effect is not produced, and, if successfully killed and 
hardened in this fluid, ova are often found to produce most satisfactory sections. It is, however, not 
reliable. Creosote is added on Kleinenberg's suggestion, but apparently without much effect. If the 
ova placed in picro-sulphuric acid maintain their normal shape, they remain four or five hours, and 
then are transferred into 70 per cent, alcohol, which is frequently changed, as it becomes stained by 
the yellow picric acid. When the alcohol is seen to be uncoloured, the ova are then ready to 
be transferred to absolute alcohol, preparatory to clearing. Emery recommends this fluid for 

* Henneguy used elder-pith soaked in alcohol and covered with a layer of collodion. 


hatched embryos, and it is certainly one of the best that can be used for killing and hardening 

Perenyi's Fluid (Chrom-nitric Solution). — This fluid kills instantaneously, and preserves 
Teleostean ova better than any other medium tried ; but in the processes subsequent to hardening, 
its action proves defective. The staining fluid is best added during the fixing process, borax- 
carmine being mingled with the solution. Pelagic eggs preserved their form admirably, and 
fixation was apparently most satisfactory, but staining was not very successful, and in the clearing 
and imbedding processes the ova shrunk, and good sections were found to be impossible. With 
some change in the mode of imbedding, this fluid would be most efficient. Whitman, however, 
states that he obtained good sections " more instructive than any obtained from eggs hardened in 
other fluids " {op. cit., p. 154). 

Chromic Acid. — The merits of this fluid for killing and fixing Teleostean eggs need not be 
insisted on. It acts perfectly ; but the long washing and difficulty of subsequent staining are 

Osmic Acid. — Alone and in various combinations osmic acid is much recommended. The fatty 
elements in Teleostean eggs, however, render it a doubtful medium, and no good results were ob- 
tained. Marshall used it for the embryos of Scyllium, which were placed in \ per cent. sol. chromic 
acid and a few drops of 1 per cent, osmic acid for twenty-four hours — thence into alcohol. 

Chrom-Platinicm. — This mixture is said to be admirable for fixing, but Whitman found that 
embryos are often rendered brittle, and contours are indistinct. It is very slow in action, but after 
washing in alcohol, staining is said to be easy and successful. 

Alcohol-Method. — A great number of ova and embryos were not subjected to special treatment, 
but were simply transferred from the tanks (sea-water) to 60 per cent, alcohol. In this they were 
killed and hardened, as ordinary museum-specimens are. Much distortion often resulted, yet some 
good sections were made of blastoderms thus simply prepared. The capsule of the egg was 
usually pierced with a fine needle to ensure entrance of the alcohol, stain, &c. 

The graduated series of alcohols was tried, and, producing less distortion, gave fair results. 
The objects were transferred from the sea-water into dilute alcohol, " Dritteralcohol," i.e., 33'3 per 
cent. ; thence in 40, 50, and 60 per cent. On account of the small size of the ova, five or six hours 
in each sufficed, extended in the stronger alcohols to ten or twelve hours. 

III. Staining. — Only alcoholic stains were used, and Beale's solution, if not too newly made, 
gave very satisfactory results. It requires long immersion, rarely less than twenty to thirty hours, 
and is apt to be diffuse, but acidulated alcohol in a short time makes it markedly nuclear. Diluted 
with alcohol, the penetrative power of this stain is increased. 

Borax Carmine (Naples formula) is one of the most successful stains — penetrating and nuclear, 
and sections are additionally valuable if, after overstaining, the eggs are placed in acidulated alcohol 
until the surplus is removed. 

Hccwiatoxylin (Kleinenberg's). — This proved less useful than might have been supposed ; no 
good sections of early blastoderms were obtained after the employment of this stain, but more mature 
tissues were very satisfactorily treated, the stain being of the most pronounced nuclear character. 

On the whole, the carmine stains are found to be the best. 

IV. Imbedding. — Prior to imbedding, the ova were finally dehydrated by an immersion for two 
or three hours in absolute alcohol, and transferred thence either into benzine, oil of bergamot, 
or chloroform — clove-oil, creosote, &c, not being found to act well. The transference was made 
gradual by the method of Giesbeecht. Turpentine succeeded the bergamot, in other cases 
a mixture of the clearing agent and paraffin followed, fragments of paraffin being added until 
finally the objects were transferred to pure melted paraffin in the usual way. Mixtures of the 
hard and soft paraffin, supplied by the Cambridge Instrument Company, were used — the proportions 
varying according to the temperature of the laboratory. Before transferring from the final absolute 


alcohol, it was found necessary in the case of certain embryos to remove the yolk. In such com- 
paratively large forms as Cyclopterus, Cottus, Anarrhichas, and Gastrostcus, the yolk became so dense 
in the hardening process that the razor of the microtome would not pass through it ; hence, by 
dissecting off a portion of the yolk-sac, the enclosed yolk could with care be removed en masse. 
Whitman {op. eit., p. .178) recommends Gastrostcus as especially suitable for sections, forgetful of the 
fact that the yolk-mass presents peculiar difficulty to the microtomist* — in contrast to the yolk-mass 
of more delicate ova, such as the cod, whose yolk is cut with ease by the razor. Ova which contain 
large oil-globules, e.g., Trigla and Molva, are not reliable for cutting, the alcohol removing their 
constituent fluid, and leaving large empty cavities in place of the globules. 

XI. Embryonic, Larval, and Post-Larval Conditions of the Food FiSHES.t 

Trigla gurnardus,\ L. — In dealing with the ova of this species, it has as a rule 
been found at St Andrews that the ripe females are considerably larger than the males, 
but whether this is due to the fact that the males, as in some other fishes, e.g., the 
salmon, become earlier mature, or to other circumstances, is at present undetermined. 
The rate of development of the embryo depends much on the temperature, thus ova 
fertilised on the 6th May hatched on the 13th day, while the embryos escaped from the 
eggs on the 6th day, respectively on 17th June and 5th July 1885. The spawning 
period of this form is thus considerable, viz., from April to June.§ 

The young gurnard, on emergence (PI. XII. fig. 1), is a glassy transparent 
form with a considerable yolk-sac, the oil-globule (og) in which is conspicuous at 
the posterior angle, and is surrounded by a thickened layer of protoplasm (p). 
Numerous round pigment-corpuscles of a dull yellow or olive colour, often apparently 
dull greenish, are scattered over the head, dorsum, and latero-ventral region, but 
they do not extend to the tip of the tail. The dorsal margin of the embryonic 
fin has finely ramose, dull yellow, pigment-spots, with a few intermingled black 
corpuscles. These proceed within the dorsal edge, and may be traced down to the body 
line, a short distance in front of the tail, finally intermingling with the branched pigment 
on that portion of the animal. A similar pigmented area occurs along the ventral fin for 
a short distance. The coloration of the pectoral fin (pf) is very striking, an arch of 
pigment-corpuscles passing across the base of the organ, which, as in the young cod, is 
now erect. Over the yolk, as already noted, many stellate yellowish and a few 
black corpuscles occur, and they often anastomose. We have seen that this 
colouration of the yolk-envelope is characteristic of certain species, the gurnard being 
one, while in others, e.g., cod and haddock, this feature is absent. Besides the opercular 
aperture, a single gill slit (?) at this stage occurs above the heart (PL VIII. fig. 8,poa), 

* Wenckebach, who killed the embryos of Perca in corrosive sublimate, and stained in picro-carmine, alludes to 
this character of the yolk — " the embryos being very small, and the yolk extremely hard in the preserving reagents . . . 
satisfactory sections are difficult" (No. 157). 

t The order of convenience only has been followed in this section. 

t Day (Commercial Fishes of Brit, p. 77) states that the gurnard probably spawns twice a year, viz., in mid-winter 
and mid-summer. If he means that each individual fish spawns twice, there would seem to be no structural grounds 
for the remark. § Mr Scott found ova of this species in the Moray Firth in January. 


and the epidermis of the cephalic region is very uneven. The heart (h) has the siphonal 
shape, and the dilated venous end is curved to the right. In some examples a large 
space — Ryder's segmentation -cavity — is present below and in front of the heart (PI. VIII. 
fig. 6,pd), while in others this space is either much reduced or is not present. In some, 
again, the pigment is less developed than in others, the former possibly having emerged 
at an earlier stage than the latter. The larva at this time hangs in the water with 
the yolk uppermost, the head being often directed downward. 

2nd day. — On the second day the pericardial wall has, in front, shifted downward, so 
that its attachment terminates anteriorly some distance below the junction of the throat 
and the yolk-sac. The latter is diminishing, and has already receded from the rectal 
bend of the intestine. A large lumen is present in the oesophagus, and it distinctly 
passes beneath the eye. The pigment-corpuscles at the margin of the dorsal fin, which 
were at first amorphous, are now finely branched. A very remarkable phenomenon is 
the shortening of the region between the pectorals and the otocysts, coincident with the 
great growth of the pectoral fins. Three branchial arches are distinctly visible, and 
have an oblique dorso-ventral direction, but the slits do not appear to open externally 
at this stage. 

3rd day. — On the third day (PL XIV. fig. 2) the chief changes are the increased 
prominence of the snout, which now projects in front of the yolk-sac, the general 
shrinking of the latter, and the very finely branched condition of the pigment-corpuscles 
in the marginal fin, pectorals, and on the yolk-sac (PI. V. fig. 2a). More pigment, of 
a yellowish colour, now occurs over the mid-brain and round the eye. The reticulation 
of the peculiar pigment-corpuscles of the yolk-sac is conspicuous (PI. V. fig. 2a), these 
bodies wholly differing in shape from those of the embryonic fin and other parts (PL 
XVI. fig. 8, those of the trunk being figured on PL V. fig. 2). The pectoral fin has 
acquired greater prominence, and its distal margin is rounded. Little change has 
occurred in the outline of the marginal fin. Surface-views still show that the oral 
region is impervious from the widely open mouth to the eye, but the lumen of the 
alimentary canal posteriorly is very distinct. The liver projects prominently opposite 
the posterior border of the pectoral fin. The urinary vesicle (uv) is elongated from above 
downward, and the segmental ducts often appear to enlarge before opening into it. The 
larvse at this time show increased activity, and jerk or dart about at intervals, 
apparently for respiratory purposes. In certain cases the well formed and active 
larvse keep near the bottom of the vessel, while the deformed examples float helplessly 
on the surface. They occasionally remain still, hanging obliquely with the head down- 
ward, and gradually descend to rest quietly on the bottom. The fine yellow pigment and 
shining oil-globule in the yolk are diagnostic features. The dead sometimes float as 
minute white objects on the surface, though generally they sink to the bottom. 

5 th day. — When five days old the gurnard measures *165 of an inch. The eyes 
have a greenish lustre, with black pigment. The ochre-yellow pigment is now chiefly 
confined to the head, yolk-sac — where the corpuscles are finely ramose, the pectorals, the 


anterior dorsal region, and the base of the mandible, but they are very sparse on the 
opercular and abdominal surfaces. In the region at the base of the abdomen black 
pigment-spots are numerous, while one or two occur on the tip of the snout and along the 
ventral margin of the myotomes. The pectoral fins form a pair of great fan-like organs 
dotted with yellowish pigment and very minute black spots, while delicately branched 
yellow corpuscles occur towards the free margin. No feature is more striking than the 
great development of the pectorals; they project almost at right angles to the body, their 
concavity being directed backward (PI. X. figs. 2, 2a). They actively move with a 
vigorous paddle-like motion, and aid effectively in progression. 

The tail now shows dorsally and ventrally three ridges which slope in the former 
case upward and backward, and ventrally downward and backward. The mandible 
remains stiff, or is very slightly movable, and as' the upper jaw projects, and the mouth 
is wide open, the appearance produced is remarkable and diagnostic. Aeration is- suffi- 
ciently provided for by this wide and rigid oral aperture, and the energetic forward 
movements of the fish. In a deformed specimen at this stage the urinary vesicle was 
large, and distended with a large number of minute highly refracting granules. More- 
over, the dorsal blood-vessel (vs) was in course of formation, since rows of comparatively 
large cells formed a definite tract beneath the notochord (n), as was also plainly seen in 
the larval ling (PI. XV. fig. 1). This specimen was apparently affected by hydrops 
pericardii, for the heart was directed at right angles from the pre-hyoidean region, and 
the venous portion formed a spindle-shaped process attached by a narrow neck to the 
ventral pericardial wall. At this latter end of the heart large rounded globules occurred, 
while the arterial portion was attached in front to the posterior part of the branchial 
framework. Probably by the dragging down of the membranous attachment of the 
venous end, its spindle-like form was acquired. The yolk is now very much reduced. 

On the following (the sixth) day, the rapid development of pigment greatly obscured 
the internal structure of the young fish. On the eighth day the premaxillary region 
sends out a pair of prominent knobs, theprecursors of the spinous ridge which is subse- 
quently formed. The anus, which has probably been open a day or two, now shows a 
distinct corrugated aperture. The rectum is often swollen, apparently with a watery 
fluid, and its strongly folded walls contract powerfully — expelling a riband of translucent 
mucus containing minute refracting (fatty ?) granules similar to that discharged in the 
tanks by the adult Cyclopterus. The mouth (m, PI. IX. fig. 5) is still rigid, but widely 
open, and the gullet leads into a pendulous, sacculated stomach immediately behind the 
liver. Thus the course of the oesophagus behind the otocysts is backward and downward. 
The gut leaves the upper border of the stomach, passes along the roof of the abdominal 
cavity, and bends downward to the anus at an angle slightly less than a right angle. 
The whole alimentary canal behind the short oesophagus is thrown into complex rugae, 
which constantly vary with the peristaltic movements of the walls. Above the cardiac 
end of the stomach, and surrounded by the hepatic folds, is the translucent rounded gall- 


During the following days the black pigment continues to increase, especially at the 
base of the abdomen. At first the radiate corpuscles are separate, but they subsequently 
anastomose and form dense patches. 

16th day (PL X. fig. 2). — The great size of the pectoral fins (pf) is the most 
prominent feature at this stage. They are in constant motion, flapping to and fro like a 
pair of fans, and the pigment (pt) on their surface is largely developed. The yellowish 
yolk has shrunk very much, and forms an irregular mass in the pectoral region, the oil- 
globule, apparently undiminished in size, still occupying a posterior position. A large 
vacant space (ss) is left in front of the rectal tract, and a similar large space (ss) occurs in 
front of the yolk. The snout is much elongated, and viewed from above is like a 
truncated cone just as in the adult gurnard. A feature of moment is the comparatively 
motionless condition of the mandible (mn). The marginal fin (ef) shows no differentia- 
tion into definite fin-areas ; it is, as compared with the breadth of the body, now 
proportionately narrower. As above noted, the pectorals (pf PL X. fig. 2a) are the 
most noteworthy feature, standing out almost at right angles to the trunk, and so well 
developed that, viewed obliquely, the young fish resembles very strikingly Pegasus natans. 
Under a lens the yellowish pigment is seen to be confined chiefly to the head, pectorals, 
and yolk-area. A few corpuscles occur along the margin of the dorsal fin in front, and 
a few also on the tail ; but the body has fewer of them than previously, finely branched 
black corpuscles alone being present. A later stage (PL X. fig. 3), about three weeks 
after emerging from the egg, exhibits much the same features — a chitinous bar being 
prominent in the premaxillary region. The next stage under observation was procured, 
along with a large number of others, while in the Fishery Board tender " Garland," and 
the specimens were slightly larger than the last stage described, viz., about 6 mm. in 
length. They clearly were the young of the same year, as they were obtained at the close 
of summer, viz., 31st August. The great size of the head generally, as well as of the 
eyes and brain, was characteristic, and especially the broad scoop-shaped snout with the 
median " bite." Behind the head the pendulous abdomen projected like the yolk-sac 
(now almost wholly absorbed) of the earlier stages. The stomach, in fact, was found to be 
greatly distended with minute Copepoda, which form the staple food at this time. The 
pectorals are even larger than in the previous stage ; while the marginal fin with its 
embryonic rays continues into the tail fin, which shows the notochord as a median 
slightly tapered axis. From this axis the rays below slant downward and backward, 
while those above lean upward and backward. Those coming from the tip of the 
notochord are short. The ventral rays are larger, and a granular opaque tract below the 
chorda probably indicates the site of the future hypurals. No cartilaginous rays are 
present. The marked downward projection of the angle of the jaw, and the lean tapering 
body behind the massive abdominal region are noteworthy features. As the specimens, 
on account of their extreme delicacy, were injured by the pressure of the water against the 
net, it was necessary at once to consign them to alcohol, and their colours were thus 
more or less lost. Large stellate pigment-spots could, however, be distinguished along 

VOL. XXXV. PAET III. (NO. 19). 6 L 


the dorsal line of the abdomen, and a linear series also passed along the ventral line of 
the body posteriorly. These tiny, though active and vigorous, forms, had already left the 
surface where their early larval life is spent, and consorted with their older brethren 
in the still water of the open sea at 25 fathoms. The next stage, as shown in these spirit- 
preparations, is about 7'5 mm. long, and the chief changes noted are as follow : — The size 
of the head has further increased, and the snout is longer ; moreover, several sharp spinous 
processes project from the occipital area, though those on the operculum are not yet much 
developed. The translucency of the head permits the brain to be fairly seen, and the 
nasal organs are clearly outlined, as well as the facial and branchial cartilages. The 
pectorals form large fan-like organs with pigment-corpuscles thinly sprinkled towards the 
tip. The rays (thirteen in number) are all united by membrane, so that the three 
filaments which are free in the adult must be separated later. While the proportions of 
this anterior pair of fins are great, the upper rays being nearly half the length of the 
body, the ventral fins project as mere buds, so that their use in progression is trifling 
when compared with the same organs in such a form as the young ling (see p. 829). 
Along the ventral margin at the tip of the notochord, which is not yet bent upward, 
three hypural elements are visible, the first being large and prominent, the last merely a 
thin band below the termination of the chorda. Cartilaginous rays now appear in the 
ventral division of the caudal, but are absent in the median and upper portions of the 
fin, in which embryonic fin-rays still occur. In the next stage, one or two millimetres 
longer (i.e., 8 mm. or 9 mm.), the hypural elements have assumed a broad wedge-shape — 
with an even edge posteriorly and slanting from above downward and forward. Only 
a short process of the notochord is free, and this part is slightly flexed upward. The 
marginal fin still continues from the dorsum to the tail, and the inferior lobe of the latter, 
with its cartilaginous rays, has increased so as to constitute its greater part ; while the 
upper lobe, with its embryonic fin-rays, has decreased in size. The great pectorals seem 
to be growing, while the ventrals are also larger, and their rays are variegated with 
black pigment. Upon the head the spinous processes are more distinct. 

When the young gurnard has reached the length of 10 mm., spines not only appear 
on the operculum, the angle of the jaw, and the facial surface, but attain some size, two 
upon the occiput being especially prominent. The long upper rays of the wing-like 
pectorals reach nearly half the length of the fish, while the ventrals show considerable 
growth, and project freely as small fins thickly pigmented with black corpuscles. Their 
length is not quite equal to that of the basal part of the pectorals, which are, compara- 
tively speaking, enormous. The posterior border of the hypurals is nearly straight (in a 
vertical direction), and the free portion of the tip of the notochord has diminished. The 
small upper lobe of the tail continues to decrease, though the delicate embryonic rays are 
still visible in it. The remnant of the marginal fin fringing the trunk shows no carti- 
laginous rays. 

When 15 mm. long, specimens present much the same features as the last stage 
(PI. XVII. fig. 5), but the nuchal spines upon the occiput are characteristically promi- 


nent. The head is large, and the body shows an evident increase in size. The first and 
second dorsal fins are still continuous, though their separation is indicated by an indenta- 
tion. Embryonic fin-rays alone are present, those of the anterior moiety or first dorsal 
being short, while the longest rays occur about the posterior third of the second dorsal. 
This is a marked feature when contrasted with the adult, the anterior rays of whose first 
dorsal fin far exceed the others in length and strength. The caudal fin is more distinctly 
separated from the dorsal above and the anal below. Dorsally the marginal fin at the 
base of the caudal almost ceases behind the second dorsal, but ventrally it is broader 
and less distinctly separable from the anal. It still exhibits merely embryonic fin-rays. 
The caudal fin is somewhat conical in shape, being broad at the base and sloping 
to the projecting median rays, and thus very different from the slightly emarginate 
adult fin. 

The hypurals form an almost vertical border from edge to edge, i.e., dorso- ventrally, 
and the notochord now barely projects from the superior angle. Above the latter several 
linear (opaque) tracts indicate the superior accessory fin-rays, and inferiorly shorter rays 
appear next the hypurals. The anal fin shows fin-rays similar to those in the dorsal 
fin. The pectorals are still of large size, the upper rays being about double the length of 
the lower. The three lowermost rays that ultimately become free filaments are webbed to 
the tip. Black pigment has greatly increased over the fin, especially distally, and a black 
margin passes a considerable distance posteriorly. The ventrals now extend slightly 
beyond the anus. Black pigment-corpuscles have increased over the head, cheeks, 
abdomen, and ventral line of the body. 

Frequently in this and the earlier stages specimens of a Crustacean (resembling the 
young of Caligus) are found fixed to the head or other regions by the long central 

When the gurnard attains a length of 17 mm., the caudal fin is separate from the 
second dorsal, and has several accessory fin-rays. It is also free inferiorly, but the 
separation is marked by a gap behind which a portion of the marginal fin runs on to join 
the caudal, where the accessory fin-rays begin. The most prominent part of the caudal 
fin is still the median border ; but the complete separation of the anal and dorsal fins, 
and the growth of the superior and inferior fin-rays, produce a great change in its 
appearance. The first dorsal is not quite wholly separated from the second, and its rays 
are considerably longer than in the foregoing stages, while the posterior rays of both 
second dorsal and anal are longer than the remaining rays in these fins. Black pigment 
is scattered over the entire surface of the pectoral fins, extending, indeed, as far forward 
as the border of the branchiostegal region. The lengthening of the body beyond the tips 
of the pectoral fins causes the latter to appear somewhat shorter. No separation of the 
three anterior filaments of these fins has yet occurred. The ventrals have grown slightly, 
and extend a little further beyond the anus. The branchiae are now much more definitely 
pinnate than hitherto, and resemble the barbs of a growing feather. 

At the next stage demanding special notice, the young fish measures about 21 mm. 


in length, and in form closely approaches the general appearance of the adult.* The 
spines on the head are, however, proportionately larger. The first dorsal is appreciably 
larger, its supporting spines stronger, and their tips project beyond the connecting 
membrane, while a deposit of pigment has appeared in its median region. The 
posterior fin-rays of both dorsal and anal fins have increased in length, so as to cover, 
when depressed, most of the space intervening between the second dorsal and the caudal. 
A row of prominent conical papillae, larger posteriorly, extends along each side of both 
dorsal fins. A series of more minute papillae marks the lateral line. By a further 
development of the parts of the tail-fin at the dorsal and ventral accessory fin-rays, the 
outline of the tail becomes less conical, and the posterior border is now, indeed, 
distinctly truncated. The caudal fin is, in fact, much longer than broad at this stage. 
The pectoral fins, though still large, appear of less size on account of the continued 
growth of the trunk, as well, probably, as from atrophy of the upper or long rays. 
The pigment-corpuscles are, moreover, definitely grouped upon the pectorals — forming 
a basal and two distal bands, the latter conspicuously colouring the expanded fin. The 
three inferior rays are larger than the others, but still connected by membraue. The 
ventrals now extend considerably beyond the shorter pectoral rays. In some examples 
several of the parasites (Chalimus-sta,g& of Caligus) occur on the cephalic and hyoidean 
regions. The pinnately fringed branchiae show greatly increased complexity. 

A specimen, 22 mm. in length, procured in St Andrews Bay, Aug. 23, 1886 (PL XVII. 
fig. 7), presents considerable increase in the pigment of the various parts, a feature 
probably connected with its life in the shallower waters of the bay, where the sunlight 
has more direct access. The pectoral and dorsal fins, and general surface of the trunk 
and head, are boldly marked ; indeed, the little fish is most vividly tinted. Moreover, 
it is important to note that the three stronger radial filaments of the lower anterior 
border of the pectorals are now separated, and during confinement, for a short period, 
the connecting membrane was observed still further to disappear, as shown in the figure. 
Though very slightly longer than in the preceding stage, the pectoral fins are propor- 
tionately shorter, while the first dorsal and ventral are somewhat longer. The appear- 
ance of the fish, viewed from above, is shown in PL XVII. fig. 6. 

When a few millimetres longer {e.g., 24 mm.), the spines on each side of the dorsal 
fins, and along the lateral line, are very distinct. A trace of the connecting membrane 
still remains at the bases of the three free filaments of the pectoral. 

Next season the young gurnards appear to reach the length of 2f to 3 inches in 
June, though others range to 4§ and 6^, but whether the latter and those reaching 
4^ and 6^ inches in May are older forms of the same season, or belong to a previous 
one, has not yet been determined. It is probable that all may be included in the 
season's growth. 

Gadus morrhua, L. — The ova of the cod are very abundant in many parts of the 

* In a specimen whose total length was 205 mm. the following proportional measurements occurred : — Head, 
•5 mm.; tail, 5 mm.; longest feeler, 5 mm.; trunk, 10 - 5 mm.; pectoral, 6"2 mm.; breadth of head, 3 mm. 


sea at both surface and bottom from March to May,* and have a diameter of '0551 inches 
or 1*375 mm. The embryo emerges from the 8th to the 10th day in April, and in 
May somewhat earlier. 

Thus those which on the 16th April presented the multicelled lenticular stage with 
the nucleated periblast surrounding the disc, next day, at 9.30 a.m., showed a still 
larger number of nuclei in this zone, which, however, at 1 p.m. had all but disappeared, 
while the blastodermic ring had increased in size. On the 18th April the blastodermic 
ring extended over a third of the surface of the ovum, and two hours later it had gained 
the equator. At 3 p.m. the keel of the embryo had deepened, and faint indications of the 
optic enlargements were visible, while at 4 p.m. they were completed.! At 10 a.m. on 
the 19th the embryo was fully outlined, with five or six protovertebrse. The blastopore 
had closed, and there were traces of Kupffer's vesicle. At 12 noon the protovertebrse 
had doubled, and Kupffer's vesicle was more distinct. The invagination of the lens had 
commenced, and the alar membrane of the embryo was distinct. 

20th April. — The eyes, otocysts, and mesenteron (which turns to the right) had all 
made progress, and the heart showed a double-celled appearance at 3 p.m. The otoliths, 
at first very small, occurred at 4 p.m., and the pectorals were outlined. 21st April. — 
The body of the embryo jerks from side to side, and the heart pulsates languidly and 
irregularly (about 3 p.m.), the contractions, however, sometimes ceasing for fifteen or 
twenty seconds. The trunk has lengthened and the caudal extremity is flexed. The 
pectorals are more distinct, and the delicate processes anterior to the fins (observed in 
most forms) still persist. The mesenteric lumen extends as far as the heart, and 
enlarges in the mid-region. The notochord is now completely crossed by intermingling 

22nd April. — The posterior region of the trunk and tail are now flexed, and the yolk 
appears to have decreased. The pectorals are well defined and pointed posteriorly, 
while the anterior margin is rounded. The liver forms a rounded process, the heart 
shows a venous end, and the pulsations are more regular (twenty-five per minute). 
Round chromatophores (black) have appeared on the head and dorso-lateral regions of 
the trunk, but they have no regular linear disposition. 

23rd April. — The eyes show pigment, and that over the body has increased. Three 
branchial clefts and the nasal pits are visible. The violent motions of the embryos 
indicate their advancement, and a few issued from the eggs. The empty capsules retain 
their spherical shape, though a rent passes two-thirds across their diameter. 

24th April. — Five-sixths of the embryos are still in the eggs. They present a similar 
appearance to the previous day, though the increasing complexity of the branchial region 
is evident, and four clefts are visible. Some of the chromatophores on the head are 

* Mobius and Heincke state that the cod in Kiel Bay spawns from January to the end of March, but in other 
parts of the Baltic, e.g. Gothland, in April ; op., cit. p. 233 (1883). 
t The temperature of the laboratory was 59° F. 


25th April. — Further changes occur in the pectorals which are bluntly lanceolate, 
and in the pigment which in the eye has a bright bronze-like hue. The urinary vesicle 
and other viscera are advancing. 

The newly hatched cod float on the surface of still water. When a current of air is 
directed against them some wriggle aimlessly about, others, probably less robust 
specimens, float helplessly in the water. The yolk-mass is often uppermost, though some 
of the stronger carry it inferiorly. In many abnormal forms, which have a slightly 
distorted or curved appearance, the yolk lies laterally on the surface of the water. 
Four pigment-patches diversify the transparent body of the young larva, one behind 
the pectorals, one towards the posterior border of the yolk, and two on the tail. The 
disposition of these bands of pigment is well seen when the larvae are placed in sea- 
water in a white porcelain vessel (PL XIX. fig. 8). The larval coloration is temporary, 
and differs in arrangement from that in the next and subsequent stages. 

27th April. — The free larvae are very active, swimming forward in a straight course 
with considerable speed. When at rest, however, they often lie on the side, or float with 
the yolk uppermost. The snout has become free from the yolk-sac to some extent, and 
the oral aperture has burst through. The otocysts have approached the eyes. The yolk- 
sac is still large, but the breathing chamber anteriorly has expanded. The distinctive 
patches of pigment can now be made out on the trunk. In several advanced specimens 
the circulation was visible, the corpuscles passing along the dorsal aorta and returning 
after traversing about a quarter of the length of the tail. 

28th April. — The circulation can be traced two- thirds along the tail, and though a 
definite branchial circulation cannot be made out, a confused movement of corpuscles 
having the appearance of a plexus occurs posterior to the otocysts. The larval cod swims 
in straight lines, and now keeps the yolk-sac inferior. 

29th April. — The general outline is altered, the upper jaw projects beneath the eye, 
and a depression divides it from the olfactory enlargement superiorly. The mandible 
extends a little beyond the upper jaw. The yolk-sac has much diminished, the folds of 
the mesenteron have increased, and the branchial system become more complex, while 
the aorta proceeds almost to the tip of the tail. 

30th April. — The dorsal median fin now begins over the mid-mesenteric region, and 
the cuticular tissues in front form an expanded cap over the head, covered with papillae. 
This is the "integumentary vesicle" or "lymph-space" of Eyder, who mentions 
homologous structures in the Spanish mackerel and other forms. He does not now 
consider this as an extension of the median dorsal fin-fold, which is never carried to the 
front of the head. It is very characteristic of the gadoids as well as of several 

1st May. — The development of the pectorals is marked, and they are slightly angular 
in front, rounded posteriorly. They are brought to the sides, and by a wriggle of the tail 
the fish progresses. 

2nd May. — The larval cod arc now about 4*5 mm. long, and though their dis- 


advantageous surroundings have diminished their vigour, they still make active forward 
movements, and often rest on the bottom. The yolk-sac has almost disappeared. An 
almond-shaped mass lies along the floor of the abdominal cavity. The alimentary canal 
appears to be shortened, and still shows the constriction at the pylorus. No anus has 
yet appeared. The urinary vesicle is unaltered. 

Ryder* states that the larval cod has the integument raised above the head, and that 
a large serous cavity or supra-cephalic chamber is formed, which appears to serve as a 
float, but the latter interpretation is doubtful. The fish swims horizontally, but when at 
rest has an oblique position, the tail pointing backward and downward. The sub- 
epidermal space is very prominent in older specimens three or four weeks after emerging, 
and they are then very strong and vigorous, usually frequenting the bottom of the tank, a 
habit inconsistent with Ryder's view just stated, and shooting rapidly through the 
water, the large iridescent silvery eyes being the feature most readily seen. They 
dexterously escape from the forceps or other instrument used for their capture, and do so 
with considerable intelligence. The pectorals are large and strong, and the larval cod 
can direct its course with great agility and speed. The mandible and hyoidean apparatus 
project considerably, and the abdomen is hollow and shrunken (PI. X. figs. 5, 5a). 
The lateral view resembles a Chinese caricature of a fish, or a malformed trout, such 
as indicated by Agassiz and VoGT,t this effect being produced by the curvature and 
size of the head. The anus is lateral in position, and has not yet reached the ventral 

It occasionally happened that favourable circumstances enabled us to rear an example 
to a somewhat later stage. Thus, for instance, one in which the yolk had wholly dis- 
appeared on the 31st May, though the length was only about 4 mm., presented a marked 
enlargement of the head, chiefly from the great increase of the hyomandibular apparatus 
and the projection of the angle of the jaw. Moreover, the upward slope of the mandible, 
so marked at a later stage, was now characteristic. When viewed ventrally, indeed, this 
formed a high wall on each side of the hyoidean region. The body was comparatively 
massive. The cephalic "vesicle" had disappeared, but the broad marginal fin still 
surrounded the fish, and in the tail fine embryonic fin-rays occurred inferiorly. A few 
also were indicated at other parts of the fin both dorsally and ventrally. Behind the now 
open vent a rounded margin appears in the ventral fin. The pectoral fins are very large, 
and show a finely radiate basal (mesoblastic) region, and a fan-like membranous distal 
portion. The snout iu a lateral view is prominent, with a deep hollow above the pre- 
maxillary region. The eyes are large, deeply pigmented, and with the bluish silvery 
sheen so well known at a later stage. Close behind the eyes are the large otocysts with 
the otoliths. One of the most interesting features at this stage is the evolution of the 
coloration of the early post-larval stage out of the four dark bands so characteristic of 
the larval form. At the stage now under consideration the little cod has only two 

* Science, vii. 1886, pp. 26-29 (fig. 1). 

t Hist. Nat. des Poissons d'eau douce, taf. 36. 


well-defined bars behind the abdomen — by a partial coalescence of the dorsal and ventral 
masses of pigment ; the others have been modified into a band of black pigment, which 
passes along the roof of the abdomen, and if somewhat younger forms be examined 
the steps leading to the coalition of the two pigment-touches are clearly demonstrated. 
Various black chromatophores occur on the head, at the angle of the mandible, and on 
the ventral surface of the abdomen. There is thus already a change of pigment, and it 
is easily seen how the post-larval colouration develops normally from the condition just 

The scanty supply of suitable nourishment and the indifferent nature of the 
surroundings (for the water in tanks is very different from the freely aerated and 
healthy oceanic water) probably retarded growth to a considerable extent. Those 
of 4' 5 mm. in length, however, were brought within a very brief distance of the forms, 
6 mm. long, caught by the mid-water net in the bay. 

Between the stage above mentioned and the appearance of the young cod in shoals at 
the margin of the tidal rocks, there has been in this country till now a blank more or less 
complete, only a stray specimen or two — half an inch long — having been captured 
in the tow-net near the surface. The observations of the last two seasons, and 
this with the large mid-water net, # have, however, advanced the inquiry within a 
measurable distance of completion. By employing the net during the winter, as well 
as during the spring, summer, and autumn, most of the intermediate stages were pro- 

For some years the efforts of one of us have been directed specially to the 
elucidation of the history of the present species in its young condition, as the account 
given by Professor G. 0. Sars for Norway was not applicable in all respects to the 
British Seas. In 1886, indeed, some remarks were made on the young stages of the 
cod, which Professor Sars had captured, at the surface of the sea, some years ago, 
in April amidst quantities of the "herring-food," viz., Calanidse, e.g., Calamus jin- 
marchicus and Temora longicornis, species which abound under similar circumstances 
in our own seas. Besides the points indicated in the paper just mentioned it may 
be noted that on the 12th of June Sars found "that the clear and undivided 
embryonic fin surrounding the whole body had already in part dissolved into the 
first and second dorsal, and a small barbel was present." On the 5th of July, again, 
they were discovered along with young haddock, shorter and stouter in shape, under 
Aurelia aurita and Cyanea capillata, as well as under pieces of Algse; and he con- 
sidered that they associated with the Medusas for the sake of the benumbed animals 
and the parasitic Hyperige. It must, however, be borne in mind that in our seas 
Hyperiae (e.g. Parathemisto) are frequently found in a free condition and in very great 
numbers. Similar young cod were found at Lofoten in the stomachs of pollack 
(Gadus pollachius), shoals of which surrounded them, chased them to the surface, 

* Vide Ann. Nat. Hist., Oct. 1886, p. 310. 


where they were thus put within reach of the gulls as well as the pollack themselves. 
On the 3rd August the shoals of young cod, 2 inches and upwards in length, presented 
the following external colouration : — Three or four parallel lines of square spots, reddish- 
brown and more or less bright, extended along the sides, which with the head showed 
an alternating silvery or golden gloss. Sars thinks that they are driven shorewards 
when 2 or 3 inches long, by wind and currents, and seek protection from the pollack 
among the Algae at the bottom. Moreover, it would appear that the shoals succeeded 
each other, since they went off as they grew older. In the beginning of October, 
having attained the length of 4 or 5 inches, they grow more rapidly, reaching in 
the middle of November the length of 6 inches, while on the 10th of December they 
measure 6 inches to 8 inches. Towards the end of winter they decrease in numbers. 
Sars states that the last fishes to appear amongst the Algae were no larger than the 
first, and that there must of necessity be a succession of shoals. Indeed, he describes 
two varieties, viz., the thickish, reddish-yellow kind, living chiefly amongst the Algae, 
and swallowing large numbers of reddish crustaceans, and a second kind of a light 
green or greyish shade frequenting sandy ground, where the Crustacea mentioned were 
rare — these thinner fishes living on Annelids and young Cotti. Towards the end of 
February he followed them further out to sea, and found them measure on an average 
12 inches, and he was of opinion that the " Algae-fish " were one year old. The 
greatest number of these "Algae-fish" (l foot long) are caught, it may be noted, in 
summer ; but towards autumn their numbers are fewer. Accordingly, Sars concluded 
that the "going out" takes place in the second year, and that three years, or at most 
four years, hardly elapse before the fishes return to their native sites as full-grown 
cod, ready to reproduce their species. Considerably larger fishes than the forms found 
in February (l foot long) he estimated at two years old, and in these the generative 
elements were found at Lofoten not to be fully developed, the smallest breeding fish 
being nearly 1 yard in length. On the other hand, he had seen young cod 1 foot in 
length in the fish-market of Christiania, which had mature roe and milt. 

Hitherto no very young gadoids have been captured in January, February, or March, 
and it is the end of April before such appear ; indeed, they are more surely obtained in 
May in St Andrews Bay. Moreover, it does not follow that the smallest always occur in 
the earliest months, for some are found in May as small as any in April. The least of 
those hitherto secured was about 5 mm., several having been captured on the 30th April, 
and others of the same size on the 19th of May and 1st June. Now the little cod reared 
in the laboratory to a certain stage are about three-fourths the length of this on the 9th 
May, and though we cannot antedate the spawning period of the cod from personal 
observation sooner than March, there is no reason to doubt the occurrence of an earlier 
issue of ova and spermatozoa in some cases ; indeed, the general variability would hold. 
This, and the differences in the rate of growth known to occur even in those spawned 
at the same moment, give us the somewhat wide range in size with which we are familiar 
in the group. 

VOL. XXXV. PART III. (NO. 19). 6 M 


A fortunate sweep of the trawl-like tow-net on the 1st June gave a complete series 
of fresh specimens, from the form just described to other stages formerly seen. The 
smallest cod were 5 mm. in length, but they were even younger than the somewhat 
stunted specimens reared in confinement. They had the two post-anal bars, the sub- 
notochordal black band, and the scattered spots on the head and jaws ; and they were 
further characterised by the greenish-yellow colouration on the head and snout, as well as 
along the dorsal region of the body, a feature so marked at a somewhat later stage ; the 
swim-bladder (which appeared to have a short or rounded form) was distinct. The tail 
and marginal fin did not differ from the stage mentioned on the previous page. Almost 
the same remarks apply to those 6 mm. in length, some of this size presenting a 
pinkish abdomen from the oil of the minute copepods they had swallowed. At 7 mm. 
the marginal fin has many embryonic rays ; moveover, the two post- anal pigment-spots 
have spread out, so that they form a dorsal and a ventral band, though two denser regions 
indicate their former condition ; a median line also occurs laterally. The yellowish -green 
tinge is better marked. 

In small forms 6 mm. long in spirit, and probably corresponding to the stage last 
mentioned (7 mm. when fresh), the marginal fin is quite continuous, commencing 
ventrally behind the well-formed anus and passing round the tail to a point on the 
dorsum a little in front of a vertical line from the vent, though in front of this a 
membranous margin projects a short distance, indicating probably a further extension of 
the fin. Fine embryonic rays are present throughout, except in the caudal region, where 
slight linear thickenings dorsally and ventrally indicate the commencement of the 
permanent rays. The pectorals are large, with a chimseroid base and a fan-like membrane 
with embryonic rays. No trace of ventrals is visible. The mandible is bent upward 
when closed at a little more than a right angle to the body, and the angle of the jaw is 
very prominent. The eye shows a notch dorsally, and a well-marked choroidal fissure 
inferiorly. A little black pigment exists on the snout and the top of head, and along each 
side of the dorsal and ventral marginal fin, while a streak also occurs in the middle line 
laterally in front of the tail. The same pigment appears in touches on the prominent 
edges of the mandible, and along the ventral surface of the abdomen. 

In the beginning of May again, and also the 1st of June, similar forms are 
encountered, ranging from 8 to 10 mm. and upwards. The youngest of these, 8 mm. 
in length in spirit (PI. XIX. fig. 2), still presents the embryonic fin from a point on the 
dorsum distinctly behind the vertical from the pectorals all round to the vent, the tail 
as yet showing no special differentiation. At points, however, corresponding to the two 
posterior dorsals and the two anal fins, thickenings — indications of the adult fins just 
mentioned — beyond the body-line are noticed, at the base of the embryonic fin. Beyond 
these rudiments the embryonic fin is unaltered. The tail forms a perfectly symmetrical 
organ, convex posteriorly, and having the notochord as a straight, tapering, and translucent 
streak in its centre, with the hypural and epiural elements disposed ventrally and dorsally, 
and so equally that the whole presents a lancet-like figure in the middle of the tail. 


A little pigment exists in the interspaces of the rays over a limited area dorsally and 
ventrally. The pectorals have a large — almost semicircular — basal region, and a fan- 
shaped series of rays distally, so that they are still powerful, but the ventrals are visible 
only as two minute ventral papillae on the throat in front of the former. The body and 
tail have increased considerably in bulk, but the head and anterior region still remain of 
great proportional size. The angle of the mandible is prominent, and the jaw has the 
larval slope upward and forward. The eye retains its great size. The black pigment 
occurs on the top of the head — on which the chromatophores are now larger, along the 
base of the dorsal, and less distinctly along the base of the ventral marginal fin, with a 
streak in the middle line of the body towards the caudal region. The only other pigment 
is in the abdominal region — from the top of the pectoral in a line downward and back- 
ward to the anus, and this for the most part is internal. Yellowish-green pigment also 
occurs here and there all over the surface, so that the animal when living presents a 
greenish translucent aspect, and it is also noteworthy that the dorsal pigment is in two 
sections on each side, thus indicating the two original spots. The eyes at this stage are 
proportionally large, as in others of the group, of a bluish silvery aspect, and with 
a dark arch of pigment superiorly. The bluish sheen is probably due to interference, 
and not to any special pigment. The abdomen has a slightly pinkish hue from the 
Crustacean food which filled both stomach and intestines. The branchiae show simple 

At a somewhat older stage (PI. XIX. fig 3) the three dorsal fins are distinct, as also 
are the two anal. It may be noted also that the first dorsal develops somewhat later than 
even the two succeeding fins, that is to say, it presents only a thickening, while they 
have rudimentary rays — for instance at a length of 10 mm. and 13 mm. In the latter 
the swim-bladder assumes a more elongate aspect. The ventrals show more evident rays, 
the growth of the body and head diminishes the proportional size of the eye. • The snout 
is longer, so that the mandible bends less obliquely upward than in the previous stage. 
The blackish pigment has increased on the lines formerly mentioned, and also at the base 
of the abdomen. While in the earlier stages the tail of the young cod presents a straight 
notochordal process posteriorly, it now (at and near three-eighths of an inch in length) 
shows a distinct upward bend apparently from the development of the hypural elements 
inferiorly. The tapering tip of the notochord issues therefore from the upper part of 
the pointed central mass, the shape of the region, however, marking the usual 
transformation caused by the shifting of the ventral margin to the posterior region of 
the tail.* 

A month later, viz., on the 1st June, considerable progress had been made in the 
growth of the young cod, which were caught both in the trawl and in the mid-water 
net, sunk 3 or 4 fathoms in 6 or 7 fathoms of water, showing that these fishes gene- 
rally seek the lower regions of the water. The length of the smallest was about ^f 

* The great length of the notochordal tip (embryonic tail) in Lepidosteus is noteworthy (Balfour and Parker, 
op. cit., p. 374). 


inch. The pigment is not yet arranged in transverse bars, but has the character 
described in the earlier stages, being chiefly grouped on the head, along the dorsum, and 
on a lateral line. Stellate pigment-spots are somewhat thinly dotted here and there on 
the sides. Moreover, in several, after preservation in spirit, the pigment-corpuscles on 
the head show a central nucleus, then a pale area, and externally a ring or border of 
black pigment, the whole presenting the appearance of minute mosaic work. The 
abdomen in all is tinted of a pale orange hue from the Crustacean food which distends 
both stomach and intestines. The same food is eaten by the small sand-eels, young 
armed bullheads, and other fishes captured with them. The ventrals are now well formed, 
and show the elongated outer rays, though these are less developed than in a subsequent 
stage. Glancing generally at the contour of the fish the origins of these fins (ventral) 
also appear somewhat further forward than in the later stages. The barbel is now 
distinct, though it is less conspicuous from length than thickness. 

As the fishes get larger (? older) there is a distinct aggregation of the black pigment 
along the sides, and the appearance of a brownish tinge in the skin on which these 
pigment-specks rest. These young cod are paler than the young green cod, from which 
they are also distinguished by the size of the barbel (which is very small in the green 
cod), and the longer snout in front of the eye ; while the appearance of the pigment-spots 
along the sides at once removes any ambiguity. Moreover, the eye of the green cod 
is somewhat larger, proportionally, than that of the cod, probably from the shorter snout, 
and the mandible in the former is longer, when each is about 1^ inch in length. The cod 
also soon shows a series of pale dots, from 4 to 6 in number on each side, along the 
dorsum, and the general habit of the fish differs quite from that of the green cod, as 
formerly mentioned.* Spirit-specimens, about \\ inch long, are readily discriminated 
from the green cod by the pigment-bars and pale areas, and the barbel, as well as by the 
general sprinkling of pigment-corpuscles over the entire area in the green cod. The fins 
in the young cod vary considerably in regard to pigment, many presenting at this stage 
a slight marginal black band, but as a rule they have much less pigment than in the 
green cod, which, moreover, shows grains of yellow pigment in the dorsal fins, and to 
a less extent in the first anal. 

Alexander Agassiz mentions and figures t two specimens, probably of the common 
cod, 20 and 28 mm. in length respectively, the former without the pigment-bars, devoid 
of a barbel, and with the median fins still somewhat continuous, the latter with long 
ventral fins, pigment-bars, and the general feature of the adult. As a rule the cod of our 
eastern shores show the characters of the adult before reaching so great a length. More- 
over, instead of simple ventral pigment-bars, the dice-like pattern of the pigment is 

The young cod which, in company with the green cod (Gadus virens), frequents the 

* Ann. Nat. Hist, Oct. 1886, p. 307. 

t Proc. Amer. Acad. Arts and Sci., vol. xvii. p. 286, pi. viii. figs. 4, 5, 1882. 

X Vide Fourth Report, Fishery Board, for Scotland, and Ann. Nat. Hist. 


rock-pools of St Andrews in June and July, often hang in the water obliquely with their 
heads downward against the current. Their food at this time, when they measure If 
inch to If inch in length, consists of copepods, larval cirripedes, sessile-eyed crustaceans 
(larval), small annelids, and Campontia, while the green cod, in addition to that food, 
feeds upon minute Mollusca, e.g., Homalogyra rotata, and various species of Ostracoda. 
The cod is less shy at this stage than the young green cod, and it is captured with less 

Viewed from the dorsum they have a general pale olive-green colour. The sides 
are iridescent, with a pretty pinkish pearly lustre. The upper surface and sides of the 
head to a level with the eyes are studded with dark pigment. A regular series of dark 
pigment-spots runs along each side of the median dorsal line to the tip of the tail. 
About eight dark blotches occur on looking at the median lateral line, and as these are 
flanked by other dark patches in the upper lateral region, they give a very characteristic 
appearance to the fish (PI. XVII. fig 8). This upper lateral region, just below the lateral 
line, shows behind the operculum nine dark spots. The first three are continued on the 
silvery belly, and then cease. The rest have connections with a series of median spots 
(five in number) in the middle line — bands, in several instances, passing from two upper 
spots to one lower median, or again bifurcating inferiorly. The ventral median line has 
on each side a band of pigment, continuous with the bars just described; but the pig- 
ment-corpuscles are less distinct than along the dorsal lines, except opposite the base of 
the vertical fins, where the pigment is quite regular, and corresponds with the base of each 
ray. The first two dorsals have the blackish pigment towards the tip best developed on 
the membrane between the rays, the basal region being pale. The third dorsal has only 
a little black pigment. A trace of pigment also occurs towards the commencement of the 
anal fin. Blackish pigment is scattered on the sides and under surface of the mandible, 
and a thin dark streak passes backward in the middle line. The eyes are of a pale 
olive-green hue, with dark specks of pigment. The upper opercular region, and the 
surface above the cerebellum, are of a pale pinkish colour, due to the blood-vessels 
and the brain beneath. The vascularity of the latter seems to be considerable. The 
opercular region and the body are silvery. The pointed teeth are very evident in the 

The later stages have been dealt with in former papers, and need not be alluded 
to at present, except in regard to Eimer's # notion that the markings in animals are 
primitively longitudinal. Now the young cod is conspicuously speckled in its earliest 
stage, and is rather pale and translucent in its next condition, the pigment which forms 
the transverse bars gradually grouping themselves on a somewhat pale surface, without a 
trace of longitudinal bands. In many other fishes, both round and flat (Pleuronectids), 
the same arrangement obtains, so that Haake had good grounds for demurring to this 
view from the study of the Australian fish Helotes scotus, which in the adult is marked 
by eight longitudinal bands, while young specimens have in addition a row of clear 

* Zoolog. Anzeiger., viii., 1885, pp. 507-8. 


transverse bands which disappear when they attain maturity. In one fish, viz., the ling, 
the post-larval stage is uniformly tinted, the next stage longitudinally striped, the third 
transversely barred, while in the adult it is uniformly tinted as in the older post-larval 
condition. No rigid rule can thus be held. 

Gadus aeglefinus, L. — The ova of the haddock are about *058 in., varying a little, e.g. 
from 1*65 mm. to 1'5 mm. The development of the embryo ranges from six days in 
June to twenty in March.* Thus a series received from Granton presented on the 
second day (22nd March 1885) a blastodermic cap - 4 mm. in diameter. It reached the 
equator on the fourth day. While the keel of the embryo indents the yolk, the head is 
defined, and everywhere shows further progress. On the fifth day the optic enlarge- 
ments are distinctly outlined. Faint indications of pro to vertebrae (four to five in number) 
appear in the anterior caudal region, and scattered black pigment-specks show on the 
sides and dorsum. On the seventh day, at 9 a.m., the blastopore had closed, but 
Kupffer's vesicle was not apparent till next day. The lenses of the eye are fully 
formed, and the heart is represented by a granular patch. On the tenth day the various 
regions of the brain were defined, with the nasal pits, the otocysts, and an opercular 
cleft ; the liver is indicated on the ventral aspect of the alimentary canal. No cavity is 
visible in the heart ; the latter pulsates on the eleventh day about ten times per 
minute, though occasionally a little more rapidly, and shows a somewhat triangular 

The pigment-spots are more numerous and more elaborately stellate on the twelfth 
day, especially on the dorso-lateral regions above the pectorals. A lateral fold arises 
behind the latter and passes along each side. The lumen of the mesenteron has notably 
enlarged next day on the dorsal side of the liver, but it diminishes very much as it 
approaches the cephalic region. On the fifteenth day the cephalic region has increased 
in size, and the body has considerably lengthened. Embryonic rays have appeared 
in the marginal fin. The heart pulsates on the seventeenth day about thirty times 
per minute. The eyes have a punctate appearance from the development of pigment, 
and the first branchial cleft is distinct. On the eighteenth day the eyes have black 
pigment. A second branchial cleft occurs on the ventral side of each otocyst. The 
liver has largely increased and projects into the yolk-sac. The pectorals show a distinct 
rim. The alimentary canal is filamentary anteriorly, and ends blindly in an enlarge- 
ment posteriorly. Three branchial clefts are visible on the nineteenth day, and the 
pulsations of the heart are forty per minute. A buccal chamber is continuous with the 
mesenteron, which has a flexure to the right of the embryo. The segmental ducts and 
the urinary vesicle are well advanced. 

The embryos emerged on the twentieth day 3 mm. in length, and with a yolk-sac 
"5 mm. in its long diameter. They attempt to progress with the yolk-sac downward, 
but at rest are inverted. The black pigment-corpuscles are grouped somewhat densely 
behind the otocysts, and extend backward a little beyond the commencement of the 

* In contrast with the ova fertilised on the 24th April, and hatched on the 3rd May — that is, in nine days. 


intestine. A line of the chromatophores passes along the infero-lateral region from the 
beginning of the mesenteron to near the caudal tip, and a few exist on the dorsal part of 
the abdominal region. A fourth branchial arch is visible. A delicate polygonal proto- 
plasmic meshwork occurs over the surface of the yolk, as in the flounder. The walls of 
the heart are thinner, and cellular strands pass backward to the liver. There is neither 
mouth nor anus. The alimentary, renal, and other organs have been further developed 
on the third day (of freedom), and the urinary vesicle sends down a fine strand of cells, 
the precursor of the urino-genital tube. On the fifth day rapid elongation of the 
skeletal elements of the mandible has occurred, the head has been raised, and the cranial 
flexure diminished. The point of the snout is now in the same line as the ventral 
margin of the liver. The abdominal pigment has increased, but there is little change 
in the rest. The oral chamber has now burst through. The otocyst presents a ridge 
growing up from the floor, and a chamber descending from the roof, the otoliths lying 
on each side of the former. A lenticular mark indicates the anterior nares. The 
mouth gapes, but only erratic movements of the parts take place. Next day the 
mandible protrudes further, and the branchial and hyoidean arches are prominent. The 
yolk-sac is oval and much diminished. On the seventh day blood began to pass into 
the heart, but the death of the embryos arrested further examination. 

The newly hatched larvae of this species are very small, about 3 mm. in length, 
and irregularly pigmented with black. They emerged in June, in about six days after 
fertilisation, and are very active when free. In a week they are difficult to see when 
resting on the bottom, and if stimulated they glide rapidly, seldom rising above the 
bottom, or at any rate rising very little, and progressing with a jerking motion, the yolk- 
sac being inferior. When at rest in the water, the head hangs slightly downward as in 
other young fishes, and in descending they wriggle a little and elevate the anterior region. 
They are chiefly recognised by the eyes, which are large and pigmented, and also by the 
pigment passing along the dorsal edge of the abdomen as well as a faint line below the 
muscle-plates of the same region. 

The post-larval stages of the haddock have hitherto escaped detection, and it is only 
when the fish reaches the length of upwards of 2 inches, with the characters of the adult 
fully displayed, that it has come under notice. Few authors allude to the very young 
stages of this form, though G. 0. Sars thought he could distinguish them amongst 
other young gadoids by their shorter and stouter form. Collktt again states that he 
found the young haddocks 7 cm. long under Cyanea capillata* 

Gadus virens, L. — The ova of this species have not been recognised in the ripe 
condition. It is stated by Kroyer to spawn in January. In its earlier stages the green 
cod probably resembles the cod very closely, and follows similar habits. When 1^ inch 
in length they come in large numbers to the margins of the tidal rocks about the end of 

* Vide Mobius and Heincke, who quote without criticism the remark of Malm that the haddock spawns in shells 
on the west coast of Sweden from January to March ; and that in the museum at Kiel is a shell-fish on which are 
ripe eggs. 


May and beginning of June, preceding the arrival of the young cod of the stage formerly 
mentioned, though perhaps not always. The characteristic features of the species, as 
distinguished from the cod of the same size (1-g- inch), have already been indicated. 
They consist of a deeper green hue all over, but especially anteriorly, and a much greater 
development of black pigment-corpuscles both on the body, head, and fins. The eyes 
also have a greenish hue, and these are proportionally larger than in the cod. The fins 
throughout are duskier from the black pigment, and the three dorsal and anterior anal 
are often marked by yellow pigment-grains. The pectorals in some show traces of two 
broad arches of pigment, after the manner of other larval forms, such as the gurnard and 
armed bullhead, though much less distinctly. The ventrals are well formed but small, 
and show no special elongation of the outer rays. When specimens of this and the cod 
are viewed side by side from the dorsum the difference in regard to pigment is striking, 
the green cod being almost uniformly pigmented from the tip of the snout backward, 
whereas the cod shows such chiefly on the tip of the snout and over the brain. More- 
over, the snout in the young cod is decidedly longer and narrower, so that with the 
distinction already noted in regard to the size of the eyes the whole facies differs. In 
profile the gape of the cod is the longer, the mandible apparently being longer, and the 
angle more pronounced. 

A curious feature was observed in those killed by a few drops of corrosive sublimate 
(in acetic acid), viz., the closely adpressed condition of the first dorsal fin. 

In somewhat older forms, which are abundant in the rock-pools in July and 
August, two varieties occur, viz., one of a pale though dull green along the dorsum and 
upper lateral regions, the other of a dark olive-green in the same parts.* 

Gadus merlangus, L. — The eggs of the whiting abound in April and May, and 
probably later. t They measure *0476 in., or about T125 mm. In an instance 
in which they were fertilised at 3.30 p.m. on April 15, 1885, the germinal cap was 
found at 6 p.m., and forty minutes afterwards the first furrow had appeared. At 9 p.m. 
segmentation had proceeded beyond the eight-cell stage, and soon sixteen were outlined, 
the nuclei in these being apparent at 9.40 p.m. On the second day, they were in the 
multicelled stage, but no well-defined nuclear zone was visible, the latter being very 
distinct on the third day. The blastoderm had largely extended on the fourth day, and 
on the sixth the blastopore had closed, though Kupffer's vesicle had not yet appeared. 
Lenses and otocysts were present. No pulsations of the heart occurred early on the 
seventh day, but later intermittent contractions took place. Finely stellate chromato- 
phores develop on the yolk-sac. \ On the eighth day yellowish chromatophores appeared on 

* Report on Trawling (1884), p. 360. 

t Day says the whiting "spawns in March not far from the shore," though what advantage the latter situation 
gives is not stated. Mobius and Heincke observe that, according to Benecke, it spawns on the Prussian coast from 
December to February, and in the Cattegat, according to Malm, from March to May. 

X Mr Cunningham considers that in the larval whiting the chromatophores are conlined to the body of the 
fifth, and are absent from the marginal fin and the surface of the yolk. His diagnosis rests on specimens captured in 
the tow-aet {Jour. Mar. Soc. Biol. Assoc, N.S., i.) In our experience the whiting is a form very early characterised by 
in yellowish pigment, which invades the marginal fin. 


the dorsal region, and on the yolk-sac, near the trunk. The cardiac pulsations were 
vigorous and regular. The pectorals were distinct. Next day the movements of the 
contained embryos were active, and some emerged at 5 p.m., most however issuing next 
day (tenth after fertilisation, viz., 24th April) : they are delicate, translucent forms, 
and swim vigorously near the surface. The pigment is characteristic (see p. 126). 

The oldest larva reared in the laboratory is shown in PL XVII. fig. 12. It is 
distinguished by its black pigment-spots arranged in a double series along the edges 
of the muscle-plates, the inner row in each case being somewhat fainter. Dorsally the 
outer row reaches forward to the mid-brain. A dense pigment-band exists in the 
subnotochordal region of the abdomen. Scattered spots of considerable size occur on the 
mandible, over the cardiac region, and on the ventral surface of the abdomen. As in 
many other forms the dark pigment abruptly ceases in front of the caudal region. The 
yellow chromatophores are distributed generally over the head, trunk, and fin-mem- 
branes. The eyes are bluish silvery, the snout is still blunt, and the mandible is stout 
and prominent. The subepidermal serous space over the head is well marked, and 
extends as far as the anal region. Three sensory organs are present in it. The otocysts 
are comparatively large. The blood-corpuscles are distinct. This example nearly bridges 
the gap to the post-larval forms. At this stage the great translucency of the species is 
noteworthy, all the organs being most clearly observed. 

The earlier post-larval stages of the whiting, viz., those immediately following larvae 
reared in the laboratory, are still somewhat obscure, though they probably closely approach 
those of allied forms, such as the cod and haddock. The characteristic nature of the 
larval pigment, however, would lead to the belief that in the brighter colours (e.g., yellow) 
early differences may occur. Such, as a rule, were lost before they came under observa- 
tion ; for all these delicate forms are dead and considerably altered before reaching the 
deck, and the same remark applies still more decidedly to those immersed in spirit. The 
pressure to which they are subjected in the large mid- water net, by the currents, and by 
the weight of crowds of Appendicularians, Medusas, and Hydromedusse, as well as Cteno- 
phores, would alone sufficiently explain this ; nor are these dangers obviated by the use 
of large wide bottles at the extremity of the net. 

So far as present observations go, the young whiting appears to be recognisable as 
such when from 9 to 12 mm. in length, examples of these stages occurring in August 
(1886). The dorsal, anal, and caudal fins have permanent rays, and the several parts of 
the two former are all outlined but not separated from each other. The pectorals form 
large fan-shaped organs, but the ventrals are minute. Groups of black pigment- 
corpuscles occur on the brain and along the sides of the dorsal and anal fins, while a line 
runs in the median ventral region of the abdomen. The sides of the body posteriorly have 
a more general sprinkling of black pigment than in the cod, which, however, it closely 
approaches. No barbel is noticeable. 

When about 15 mm. long the species is distinguished by a more abundant covering 
of minute black pigment-specks along the sides of the body and on the fins than in the 

VOL. XXXV. PAET III. (NO. 19). 6 N 


cod, and by the greater length and diminished depth of the first anal fin. The pigment- 
specks are still present in the median ventral line of the abdomen. These characters are 
better marked at 18 mm., the black pigment-lines at the bases of the anal fin-rays being 
especially characteristic when contrasted with the young cod, in which a median line of 
black pigment proceeds from the centre of the tail forward to a point above the middle 
of the first anal. No barbel is present, and the myotomes are more closely arranged than 
in the cod. 

At the length of 20 mm. the first anal of the young fish assumes the adult characters, 
and a small papilla now indicates a barbel. The pigment along the dorsal edges is much 
more developed than along the ventral. The general and minute flecks of black pigment 
are very characteristic at 24 mm., and the barbel has increased in size. The denser 
dorsal pigment, moreover, has spread downward over the sides, but in the preparations is 
uniform ; and no dappled condition was noticed when fresh. 

Between the foregoing and a length of 28 mm., a decided change takes place in the 
region of the pigment last mentioned, viz., a tendency to form separate touches along 
the dorsum, somewhat after the manner of those in the cod. These dark touches are con- 
fined to the dorsal region, though in some a few bars occur at the base of the tail. The 
fish is now minutely flecked with black pigment all over the sides, head, snout, and fins, 
a few large corpuscles appearing in the dorsal and the hyoidean regions — the silvery sides 
and under surface of the abdomen alone being free from them. In general outline it 
approaches the adult. The shortness of the snout readily separates it from the cod — 
without reference to the first anal fin. A slight duskiness exists above the base of the 
pectoral, but no definite spot. The tips of the ventrals reach fully to the vent. The row 
of pigment-spots usually disappears from the median ventral line at the length of 30 mm. 
The barbel is small but distinct at this stage. 

The young whiting at 34 mm. presents the following features when contrasted with 
a cod of the same length (in spirit). Externally parasitic Caligi are generally more 
abundant in the cod. The median dorsal fin is less abruptly elevated than in the cod, 
and the first anals diverge widely. The body of the whiting is more neatly rounded and 
more plump than that of the cod, which often has a protuberant abdomen. This outline 
in the whiting is probably due to its earlier maturity. Though a smaller fish it issues 
from an egg somewhat larger than that of the cod. The pigment-specks closely cover 
the sides of the body of the whiting and the membranous webs of the dorsal fins. The same 
pigment is continued forward on the head. The pigment at the bases of the caudal rays 
is more distinct in the whiting, and the lancet-like caudal termination of the body is 
longer in this species. Moreover, the myotomes are coarser in the cod, and the surface 
has little of the dappled silvery sheen of the whiting, apparently from the somewhat 
more advanced condition of the scales in the latter. The finely stellate black pigment- 
corpuscles are larger in the cod, and instead of the general specks of the whiting, they 
are grouped in blotches over the surface, with intermediate pale regions, and the head 
and neck are much less covered with pigment. Both pectoral and ventral fins of the 


cod are shorter than those of the whiting, the tips of the latter leaving a considerable 
interval between them and the anus. This abbreviation of the abdomen coincides with 
the very long first anal fin, and is as characteristic of the adult as the young. The snout 
of the whiting is shorter and broader than that of the cod, and its depth is greater. The 
long barbel of the cod contrasts with the short process in the whiting. 

At 54 mm. the pigment has increased, and the elongate tips of the ventrals pass 
beyond the anus. The barbel is distinct but small. It is interesting that no young 
whiting of this and previous stages has been seen without a barbel, yet Mr Day and 
other authors do not allude to the subject, apparently considering that the young agree 
with the adult forms in this respect. Young whiting, between 3 and 4 inches in length, 
have more than once been observed with a distinct barbel, indeed, a stronger statement 
may safely be made, viz., that at 3^ inches some present the barbel, others do not. 

The Ling (Molva vulgaris, Flem.). — The ova of the ling measure - 066 to "0916 in., 
or about 1*08 mm., the oil-globule being T 4 5 of that size. They were fertilised at sea 
on the 27th April,* at 12 noon. When received at the laboratory at the forty-eighth 
hour, they were in the biconvex morula-stage. They appear to be more delicate than 
the ova of the cod and haddock, and many were collapsed, the contracted globe of 
yolk carrying the oil-globule in its wall of protoplasm away from the inner surface 
of the zona radiata. The nuclei of the periblast were about one-third the diameter 
of the blastodermic cells. The zona is not so soft and tough as in the cod and 
haddock, but shows greater resistance, bursting rather than collapsing under pressure. 

Third Day. — About a fourth of the yolk is covered by the blastoderm, and the 
rim is broad and distinct. On the following day three-fourths of the yolk are en- 
veloped, and the shield is outlined ; some show metameric segmentation in the middle 
region of the trunk. In the median line of the body are a number of clear protoplasmic 
vesicles between the embryo and the yolk-surface. In many, the blastopore is closing, 
the optic vesicles are contracted off, and the notochord ends abruptly in the pectoral 
region, but terminates indefinitely at the caudal end ; fifteen or sixteen somites can be 
observed, but the three regions of the brain can barely be discerned. The envelope of 
the yolk (blastoderm) is dotted with pale neutral-tinted corpuscles of various angular 
shapes which send out processes. The blastoderm shows a double contour (probably 
epiblast and hypoblast) as it passes off on each side of the embryo. 

On the sixth day the lenses are in process of formation, but cannot be fully made 
out. The neurula is well defined in the cranial region and has a marked keel. The 
large cells of the closed rim of the blastopore persist at the posterior end of the fissure 
between the embryo and the yolk. In other Gadoids, these have generally disappeared 
at this stage. The blastodermic shield is reduced to a mere film on each side, but the 
peculiar fan-like mass of cells and protoplasmic threads in front of the pectoral region 
(as in other species) has larger and more definite cells than usual. The clear 
corpuscles of a neutral tint scattered over the yolk near the oil-globule are still present. 

* The ling is said to spawn on the Skagerrack in May. 


Rudely stellate (i.e., with short rays) black chromatophores appear on the seventh 
day (May 4), on the dorsal surface of the trunk. The vesicles on the ventral surface 
still persist along with Kupffer's vesicle. About thirty somites are visible, and the 
caudal plate rises prominently upon the yolk. The otocysts are also present and have 
a circular outline. The heart is being differentiated from a protruding mass of cells 
below the otocysts, and folds immediately behind indicate the mesenteron. The oil- 
globule projects from its pocket in the yolk, externally having a covering of blastoderm. 
A central fissure occurs in each eye. 

Eighth Day. — The tail is well formed and is laterally flexed on the yolk. The finely 
branched chromatophores form two somewhat regular dorsal lines, and five or six solitary 
spots also occur over the yolk, in its outer envelope. The cardiac pulsations are faint. 
The nasal bulbs are distinct, and the lenses of the eyes fully formed. Very delicate 
round pigment-spots of a pale greenish-yellow colour appeared about noon, giving the 
ovum a slightly greenish tint to the naked eye. These greenish-yellow corpuscles were 
thickly scattered at the ventral margin, and especially on the marginal fin, almost to 
the tip of the tail. Each segmental duct ends in a space above the end of the intestine, 
and the anal tract sends a protoplasmic tube partially across the tail-fin. The lumen 
has an external opening on one side of the caudal fin-membrane. 

Next day (6th May) the embryos (PI. XIII. fig. 4) emerged, though some which 
had been isolated in a small quantity of sea-water in a room escaped the previous day. 
They measured about 3 mm., the yolk being 1 mm. in its long diameter. They appeared 
to be delicate, many lying on the bottom, while the more active floated in the reversed 
position near the surface, and were able to wriggle a little. The liver appears to be 
further back than in the other Gadoids examined, the distance being nearly one-fifth the 
length of the head and trunk (excluding the tail). The fine lumen of the mesenteron 
extends to the opening formerly indicated. Anteriorly it ends as a fine fissure behind 
the heart. About fifty myotomes are marked off, and the caudal trunk terminates in a 
slight enlargement. The otocysts have thick walls, and just in front of them are two 
clefts. The heart slowly pulsates. The finely stellate chromatophores in the caudal 
region seem to correspond in number to the lines of the metameres. 

The larvae on the 7th May presented certain peculiarities at the tip of the 
notochord very distinctly, viz., a slight enlargement followed by a constriction, then a 
large swelling in front of the terminal knob. The lumen of the intestine was slightly 
increased, though still smaller than the mesenteron proper. The urinary vesicle (which 
communicated with the rectal portion of the canal) showed continual movement of its 
walls. The yolk-cortex has receded considerably from the outer envelope (blastoderm) 
leaving a large extra-vitelline space, and the oil-globule (og) is also lifted away from 
the outer layer. The distance of the heart and the oral region from the liver is still 
marked. The heart, on the 8th May (PI. XVII. fig. 9), was much flexed, asuming an 
S shape, but no definite wall to the pericardial chamber was visible. The yolk has 
diminished. On the 10th May large ramifying chromatophores occur over the yolk, 


their long zig-zag processes being characteristic. Two or three extend over the whole 
yolk- surface. The larva now measures 3^ mm. ; from the snout to the anus ly 1 ^ mm., 
the tail being thus very long, viz., 2 T ^ mm. The lower lobe of the caudal fin appears 
to "be larger than the upper. The envelope of the oil-globule is thickened, and pigment 
appears in it. The greenish-yellow pigment-corpuscles are more numerous, and have 
a well-defined oval outline. These corpuscles are clear and homogeneous in vigorous 
examples, but they become granular in moribund and decaying forms ; moreover, while 
the black chromatophores are elaborately stellate these remain amorphous or rounded. 
The larval fishes shoot upward from the bottom of the vessel, and strive to reach the 
surface with more or less success, then, hanging head downward, sink slowly to the bottom. 
In their upward course the yolk-sac is inferior, but when the fish is motionless it turns 
uppermost and the fish descends. 

On the sixth day after emergence (12th May), an indentation passes across the 
nasal region, but in the earlier part of the day the mouth is not yet open, though a 
fold of epiblast hangs like a curtain on each side. Later in the day the mouth appears 
as a lenticular slit, cells defining its upper and lower margins. 

At the end of the first week of freedom (13th May) the larval fish measures 3^ 
mm. (PI. XVII. fig. 10). The mouth is freely open. The eyes are deeply coloured 
with black pigment. An opercular fold has grown over the cleft, leaving a fissure 
behind and below the otocysts, which are now spacious and thin walled. The pectorals 
still have a horizontal attachment, but can be elevated and depressed; and they show 
radial thickenings (fin-rays). The yolk has much diminished, and has five or six large 
stellate black chromatophores, but these do not extend beyond the yolk proper (in the 
cortex), whereas the yellow spots occur all over the integument. A pericardial wall 
appears, and the endocardial surface is rugose. The anus now opens (?) on the lower 
margin of the fin, and the space between the anus and the oil-globule is large, as the 
latter has been dragged forward by the wall of the diminishing yolk, but the globule itself 
does not appear to be much smaller. Large black chromatophores occur over the mid- 
brain, and a row of them begins near the root of the pectoral, and extends along the 
dorsal region, ceasing above the anus. At the latter a line commences along the upper 
margin of the notochord and ends a short distance from the tail, extending over § of the 
caudal trunk. At its termination, a confused mass of elongated chromatophores trends 
from the margin of the muscles outward over the tail-fin. A similar mass passes ven- 
trally. A concentration of black pigment also occurs on the dorsal surface, behind and 
above the otocysts. 

On the 30th August 1886 very young examples of the ling (PI. XVIII. fig. 3), about 
8 '5 mm. to 9 mm. in length — resembling Phycis, were captured. Along the dorsum a 
slightly greenish tint was observable, with minute scattered black pigment-spots. Two 
well-marked black bars pass behind the abdomen, one a short distance posterior to the 
latter, and another in front of the base of the tail. They are best developed ventrally, 
and do not reach to the dorsum, while a pale brownish hue surrounds them. Under the 


anterior end of the mandible as well as on the summit of the head black spots occur. A 
most striking feature is the extraordinary length of the ventral fins (vf), three of the fin- 
rays in each being very long, while the fourth is shorter. The fins are of an ochre-yellow 
colour along the rays, with specks of black pigment scattered over the inter-radial mem- 
brane. The iris, like that of the eye of the whiting, is of a pale sky-blue. The notochord 
passes almost in a straight line backward to the tip of the tail, and the caudal fin is con- 
tinuous with the unbroken marginal fin dorsally and ventrally. The great development 
of the ventral or permanent rays, however, slightly pushes the tip with the embryonic 
radial striations upward. The hypurals, two of which are very distinct, are developing 
inferiorly, and the epiurals dorsally, but they have only slightly affected the direction of 
the notochord. The early development of the upper caudal rays in this form is of interest, 
as it is in marked contrast with such forms as the Pleuronectidae in which the inferior fin- 
rays alone appear. 

The head and the eyes are disproportionately large, and the prominence of the 
hyomandibular apparatus, as well as the size of the mandible, gives to the jaws a massive 
character, just as in the cod. The angle of the jaw is especially marked, projecting 
prominently inferiorly — rather behind a vertical line drawn from the centre of the lens. 
The hyoidean apparatus, and subsequently the whole facies, the opercular structures, and 
branchiostegal rays, are remarkably developed. The pectorals (pf) have short fleshy 
bases with fan-like expansions of fin-rays of moderate length, not unlike the condition in 
the adult. The barbel can barely be distinguished. The alimentary canal has a com- 
paratively simple course, the capacious stomach bending to the right, whence a wide and 
straight intestine passes backward nearly to the anus, then bends forward, doubling again 
behind the stomach on the left side, before proceeding straight to the anal opening. 

The next observed (on 21st July 1887) was about 15 mm. in length, three occurring 
in the mid- water net in a haul at 22 fathoms. The pigment and other characters of 
these do not require special mention. The yellow pigment is at once removed by 
alcohol, but the black remains. 

On 31st August 1886 examples of the next older stage (PI. XVIII. fig. 4) were 
obtained off the Isle of May. The dorsum again was greenish, and a similar pale tint 
existed over the trunk and tail, while along the sides of the latter black pigment occurred. 
The long ventrals extended more than one-quarter the length of the body, three rays 
being especially distinguishable for their size, while three rudimentary rays were present 
at the base. Their colour is similar to that of the last stage. The pigment of the 
body, especially that of the two black bars above described, is now more diffuse and 
continuous, the bars being however indicated by two isolated dorsal bands. The 
blackish pigment in front of the ventrals is more definite, forming a broad arrow or 
A-shaped figure, and at the tip of the mandible, on each side of the symphysis, a band 
occurs, and a trace also is distinguishable in front of the barbel. Dorsally, a slender 
stripe exists along the premaxillse, and the pigment on the cranium is better defined. 
All these external features were, however, already indicated in the yo\mger examples. 


At this stage the ling measures about 20 mm. in length, and the differentiation of 
the first dorsal is complete, its position being as in the adult. The relations of the 
second dorsal fin are similar, as is also the case with the anal and caudal, the 
approach to the adult condition being marked. The tail, however, is more ovoid in 
shape than in the adult. The pectorals (pf) are broad dorso-ventrally, while the ventrals 
appear to be less advanced, that is, more directly under the pectorals than in the adult. 
The development of the hyoidean and opercular structures alters the outline of the 
angle of the mandible. At this stage the parasitic young Caligi also occur on the ling. 

The next stage observed was a specimen 3^ inches long, which had been stranded in a 
pool on the sands in the middle of December.* The fish is now boldly striped longi- 
tudinally ; thus an olive-brown band passes from the tip of the snout in a line with the 
middle of the eye straight backward to the base of the caudal fin-rays. The pale ventral 
surface bounds it inferiorly, while dorsally a stripe with a beautiful opaline lustre runs 
from the tip of the snout over the eye backward to the base of the caudal rays. The latter 
band is opaque white on the tail, and it gives the fish a characteristic appearance. The dorsal 
fins are well marked, the first presenting a distinct black speck posteriorly, and another 
black pigment-patch occurs at the end of the last division. The dorsal line from the 
brain backward is distinguished by a narrow edge of dull orange or pale olive, and this 
brings out in relief the colours formerly mentioned. The little ling is thus a longi- 
tudinally striped form, and in strong contrast with the tessellated condition of the young 
cod. The barbel is proportionally large, and is borne by the fish horizontally, i.e.. 
projecting in front of the snout. 

At a later stage, viz., from 8 to 9 inches in length, when it abundantly 
frequents the rocky margins, the ventrals show three free filaments, the first shorter 
than the second and third — which are nearly equal. These filaments in the previous 
stage (3 inches) are worn off in confinement, indeed all the fins are frayed. The 
change which ensues at this advanced stage has been formerly described by one 
of us,"}" and may be summarised in a specimen 7f inches long as follows : — The 
fish is now boldly and irregularly blotched with brown, both dorsally and laterally, 
the region of the white stripe being indicated by the pale and somewhat scalloped area 
dividing the lateral from the dorsal blotches. Fourteen or fifteen of the latter occur 
between the pectorals and the base of the tail ; they are separated by the whitish areas, 
which thus assume a reticulated appearance over the anterior dorsal and lateral regions, 
and both kinds of pigment invade the dorsal fins. The original dark greenish band is 
more or less evident from the tip of the snout to the posterior part of the operculum, but 
thereafter it is lost. The tail has a pale border, with a dark brownish belt of consider- 
able breadth, and a few black touches in it. A broad white streak exists in the upper half 
within this, but is feebly marked inferiorly. The black pigment is largely developed in the 

* Third Annual Report, Fishery Board for Scotland, p. 62, 1885. Another example of the same size has since 
occurred in March. 

t Fourth Annual Report, Fishery Board for Scotland, p. 209. 


brownish belt along the inferior margin. The black spots on the posterior part of the 
first and second dorsals are very distinct, and the dark belt of the anal is densest at the 
posterior end. In life the whitish streaks often have a bluish appearance. 

The remarkable length of the ventrals in the post-larval ling resembles the condition 
described by Alex. Agassiz, in Onus* and in Motella. 

Motella mustela, L.t — The ova of this species abound in the sea from March to May, 
and those in the tanks shed their ova freely in April. The unimpregnated egg on its 
escape has a diameter! of 73 mm., the measurements given by Mr Brook (No. 31) ranging 
from "655 to 731 mm. The hyaline capsule is slightly corrugated, and the entire yolk- 
surface presents a series of minute oleaginous particles. Mr Brook's larvae emerged 
on the fifth or sixth day, but at the laboratory the development was less rapid at the 
beginning of May, probably from the much colder surroundings. Thus ova, in which 
the blastoderm had enveloped two-thirds of the yolk (probably more than three days 
after impregnation) on the 4th May did not emerge till the 11th May. Nine proto- 
vertebrae were visible on the 5th, and the blastopore closed on the 6th May. The 
optic vesicles were well defined, but the otocysts did not appear till 5 p.m. of this day, 
the first sign of their cavity being a very fine slit. Brook gives the length of the 
newly hatched larvae at 2*25 mm. About the sixth or seventh day after hatching the 
mouth resembles that in the young plaice, the lower jaw projecting very much 
(PL XVII. fig. 2). An oil-globule occurs in the small portion of the yolk still remaining. 
The marginal fin is finely fibrous, and in the caudal region fine threads stand out 
in the moribund animal. The notochord shows very large cells, those in the tail 
being rounded and forming a single linear series, while the anterior are smaller, more 
numerous, and irregular. An interesting condition of the termination of the neurochord 
is seen at this time, for it exhibits a distinct lobular dilatation (ne) having a fine central 
canal (mc) which can be traced a long way forward. This terminal nervous enlargement 
(ne) projects beyond the end of the notochord (ne) (PL XV. fig. 4). The skin has a very 
irregular surface many from granular papillae. Three days later (May 11th) most of the 
embryos had died, as they are somewhat delicate forms ; but the survivors (see fig. 2, 
PL XVII.) show a beautiful iridescent area behind the pectoral fins, probably from the 
swim-bladder. The yolk-mass has been absorbed and the median dorsal fin has diminished. 
The mandible (mn) is still prominent. 

Proceeding to consider what may be called the post-larval stages, procured in St 
Andrews Bay, we note that Alex. Agassiz,§ in one of his papers on the young stages 
of " Osseous Fishes," in which he made known the remarkable development of the ventrals 
in a form doubtfully regarded by him as a Motella, speaks of it as Motella argentea, 
Rhein., though he added that it might be a species of Onus described by Collet. 

* Op. tit., p. 273. 

t The egg and larval form figured by Mr Cunningham (op. cit., p, 105, pi. vii. figs. 3, 4) evidently belong to 

X In parts of an inch their diameter is "0283, and the oil-globule - 0033 or less. 
§ Proc. Araer. Acad. Sti. and Arts, vol. xvii., July 1882. 


There is no uncertainty, however, with regard to the genus of the form about to be 
described. It is clearly a Motella, though more probably M. tricirrata than M. 

The youngest stage captured in the large mid-water net at the end of August is 
6 mm. in length (PI. XVIII. fig. 6), and the embryonic fin (ef) is still connected with 
the base of the tail both dorsally and ventrally, the specimen being apparently about 
the stage of Agassiz's, pi. vii. fig. 6. The body of the British fish is, however, 
proportionally shorter and deeper. The head is large and the snout blunt, the high 
arch formed by the gape being noteworthy. The mandible is large, prominent, and 
protrudes somewhat in front of the snout. The hyoidean region is well developed, 
though the branchiostegals are indistinct. The abdomen (abd) is very prominent and 
large, and has a silvery iridescent sheen superiorly. A trace of the choroidal fissure 
persists beneath each eye. Numerous blackish pigment-corpuscles occur over the 
brain; but only an interrupted line extends along each side of the anterior half of the 
dorsal fin. The pectorals (pj) form large fan-shaped structures directed upward, and 
the ventrals (vf), which are blackish in hue, are of great size ; but instead of arising 
considerably in front of the pectorals, as in the adult, they spring by a pale base only 
a very short distance in front of the pectorals. These long black fins are about one- 
third the length of the fish, and when first seen with the naked eye they resembled a 
pair of powerful black spines, for the protection of the tumid abdomen. Four of the 
rays, as in the American form, are largely developed. The body is of a general pale or 
slightly silvery hue in the preparation studied, and the stomach contained minute 
copepods. A somewhat silvery form, only 2 mm. longer (viz., 8 mm.), shows the dorsal 
and anal fins almost separated from the tail. The head is now better developed, and 
the delicate branchiostegals are very evident. The pigment over the brain is very dark, 
and a dotted band proceeds from this region backward for some distance on each side 
of the middle line. A group of pigment-specks also occurs laterally below the posterior 
part of the dorsal fin. From the elongation of the body, the ventrals do not seem to 
be so long. This example was procured on the 21st July at 22 fathoms, but the same 
stage has been seen at the surface. In young forms, apparently pertaining to Motella 
mustela, the dark ventrals show dull yellowish rays. 

The next stage (PI. XVIII. fig. 5) reaches a length of 10 mm., and the examples seem- 
ingly belong to the same species. A silvery hue predominates over the cheeks, abdomen , 
and partially on the posterior region of the trunk. Over the brain and along the dorso- 
lateral line the black pigment is more abundant, while a blackish spot occurs a short 
distance in front of the tail. The continuity of the dorsal (df) and caudal fin (cf) is 
less prominent though still present, and the tail is more elongated — tapering slightly 
towards the tip. The arch of the mouth is still high, though the forward growth of the 
premaxillary region renders it less conspicuous. The black coloration of the ventrals (vf) 
is confined to rather less than the distal half of each fin, and the length of these organs 
is proportionally shorter in relation to the body of the fish. * The eyes are really larger ; 

VOL. XXXV. PART III. (NO. 19.) 6 O 


but the abdomen is less tumid than in the last stage. Specimens slightly older than 
this are represented by Mr Couch (Brit. Fishes, vol. iii. pi. cli. and p. 113) as 
Thompson's midge, and were referred by the late Dr Gray to the genus Coryphcena, 
probably from the remarkable development of the ventrals. 

Specimens 1\ mm. longer than the last described, viz., 12*5 mm. in length, show a 
large increase in the amount of black pigment on the dorsum, where it now gives rise 
to a mottled appearance, extending over the sides and tail. Only a few corpuscles exist 
near the ventral line behind the abdomen. The pectorals have increased in size and 
strength, whereas the ventrals, though still of extraordinary dimensions, are now only 
about one-fourth the length of the body, and are tipped with deep black, while the 
remainder (|) of each fin is very pale in colour. The sides are silvery almost to the base 
of the tail. In many of the specimens a parasite like a young Caligus projected from the 
branchiostegal region. The youngest examples of Motella above described occurred in 
32 fathoms water off the Isle of May, about 7 fathoms from the bottom, the others 
were obtained in the same region in 25 fathoms water and about the same distance from 
the bottom. 

When M. mustela reaches 24 or 25 mm. in length, the general silvery hue is marked, 
only the dorsum of the head and body being brownish. The five barbels are distinct, 
and the tips of the ventral fins do not project behind the pectorals, though their bases 
have now advanced considerably in front of the former. The eye remains comparatively 
large. The specimens of this size were obtained by the surface-net in Lochmaddy. 

At 29 or 30 mm. many of the adult characters have been assumed, the brownish- 
black pigment having spread over the upper lateral regions. The tips of the ventrals 
scarcely reach those of the pectorals, the three anterior rays being furnished with long 
sensitive tips. The abdomen and lower lateral regions are silvery. 

The older Motellce obtained are characterised, as Mr Day observes,* by their very 
bright silvery sides and dark bluish-black dorsum. The black axillary pigment occurs 
in most of these, but it varies in intensity. They range from 26 to 40 mm. That 
which most nearly resembles M. tricirrata possesses a pair of very short barbels or 
papillse in the premaxillary region, but sometimes one is indistinct, and they probably 
disappear during the subsequent stages. The ventrals extend about as far back as the 
tips of the pectorals, but their bases are considerably in front of the latter, the second 
ray being the longest of the three specially developed. All the black pigment has now 
disappeared. Contrasted with M. mustela of the same length, the eye is somewhat 
larger, and the space between them narrower, while the barbels are shorter. The first 
dorsal appears also to be somewhat shorter (from before backward). The free rays of this 
fin are characteristic in all the species. 

Two examples from the surface of the sea, south-east of the Isle of May, present 
only a single median barbel on the upper lip. Both show axillary black pigment, and 
in other respects correspond with the foregoing, except that the median barbel on the 

* Op. cit., p. 312. 


upper lip is longer, as also is the barbel on the mandible, while the snout is less pro- 
longed, the latter character being indicated by Mr Day. These examples measured 
27 and 38 mm. respectively, and corresponded with Motella cimbria. 

Unknown Egg with Oil-Globule (f). — A small egg, measuring '034 by "035, with a 
single oil-globule, and in the earlier stages agreeing with Motella, was captured by the 
trawl-like tow-net on the bottom in the early part of May and for some time thereafter. 
As soon, however, as the pigment appeared in the embryos its distinction from the 
common species (M. mustela) was evident (PL V. fig. 4). The reticulated cellular 
appearance of the contained embyro and yolk was another marked feature. 

After extrusion the larva (PL XVII. fig. 4) measured about T Vth of an inch, and was 
characterised by the presence of yellowish pigment along the marginal fin dorsally and 
ventrally, blackish chromatophores occurring amongst the rest. The tip of the tail, how- 
ever, remains uncoloured. The general surface of the body, head, and yolk-sac is dotted 
with yellowish pigment, and a few black chromatophores are present on the yolk and oil- 
globule. No pigment appears in the eyes. The oil-globule is situated inferiorly distinctly 
behind the middle of the yolk-sac, but a considerable interval exists between it and the 
posterior border of the latter, thus distinguishing it at once from the egg of Motella 
mustela. Moreover, the entire surface of the larva is covered with a somewhat coarse 
reticulation of cells with nuclei, which do not occur in the centre of the cells, but at their 
margins. On the third day after hatching the mouth had not yet opened, and the only 
new feature was the more general distribution of the yellowish pigment. 

This larva was kept until the yolk and oil-globule had wholly disappeared. The 
chief change was the more conspicuous nature of the yellow chromatophores along the 
margin of the dorsal fin. The head is of a deeper yellow from the pigment over the 
brain, and the hoc\.j has many minute yellow chromatophores mingled with black. The 
pectorals are tipped with yellow, and have the streaked mesoblastic basal region. The 
eyes are greenish silvery. The mouth is widely open. At this stage it somewhat 
resembles a pleuronectid. 

An unknown larval fish (e) procured in February, and elsewhere # described and 
figured, approaches the preceding group (Gadoids) in the large size of the silvery eyes, 
which abut close on the maxillary border. At 10 mm. the notochord is straight, and 
embryonic rays occur in the tail. A marginal fin occurs along the ventral edge of the 
abdomen, which has a small yellowish oil-globule beneath the liver in front. Five black 
chromatophores occur over the head. 

Young Pleuronectid^. 

Hippoglossoides limandoides. — The long rough dab seems to spawn early in the 
season, for during the trawling expeditions ripe specimens occurred towards the end of 
March ; and some seemed to have discharged all their ova on the 21st March. Com- 

* "Pelagic Fauna," Seventh Annual Report, Fishery Board for Scotland, 1889, p. 263, pi. iii. figs. 5-7. 


paratively small specimens are productive. Northern writers give the end of winter as 
the spawning period. 

After the stage in which the young of this species and Pleuronectids generally re- 
semble the larval condition of other fishes, they begin to exhibit an increasing depth of 
the body, disproportionate to their length. In the earlier stages, when about 4 '5 mm. in 
length, this flattening and depth of the body are diagnostic. Towards the tail an 
abrupt narrowing occurs, and the slender embryonic tail proceeds therefrom as a tapering 
straight process bordered by the embryonic fin, which runs from the head dorsally all 
round to the anus. The rays are longest at the base of the slender caudal process. 
Another feature of moment is the ventral projection of the abdomen, for it extends much 
beyond the line of the body as a prominent swelling. As aids in diagnosing the mutilated 
young flounders of this stage are the proportionally larger eyes in the young round fishes, 
and the structure of the tail ; the depression of the snout between the eyes is also a 
noteworthy feature. The eyes, it need scarcely be mentioned, are quite symmetrical, as 
in other fishes. 

The most prominent feature in the next stage is the thrusting upward of the terminal 
caudal " whip" by the development of the hypural elements and the inferior true fin- 
rays. The ventral margin is also often finely dotted on each side with black pigment. 
The hypural cartilages so largely increase that they form a deep vertical boundary to 
the tail, the terminal (notochordal) process being bent upwards, and appearing, when 
viewed externally, as a slight filament. The depth of the body at the base of the tail 
has greatly increased. The left eye now shows a tendency to move forward and upward, 
and a slight twisting of the frontal region is discernible, so that the symmetry of the head 
is no longer perfect. Small lateral buds indicate the ventral fins. 

The most advanced specimens measured about 13 mm., and when one was placed 
on its side a small part of the left eye was visible above the margin of the head. 
Moreover, that eye was slightly anterior to the right eye, and its axis was directed 
somewhat forward. On the right side four black pigment-spots were situated at the base 
of the interspinous bones, and the same number, besides specks on the body posteriorly, 
occurred along the ventral region. On the left side only two were visible along the dorsal 
line, and a few scattered specks along the ventral, as well as on the posterior part of 
the body. The general outline of the body strongly suggested that the species was no 
other than the long rough dab, but the mouth seemed to be similar to the common dab. 
This latter feature may, however, readily alter afterwards. The dorsal and anal fins are still 
joined to the caudal by a marginal membrane without rays. This form ranged from 5 mm. 
to about 13 mm., and was captured in the mid-water net at the end of August. Next 
year (1887), however, similar specimens were procured towards the end of July, and one 
reached 14 mm. in length. Their distance from the shore, and the depth of water, 
besides their structural features, gave grounds for connecting them with the species 
mentioned. The young of the common flounder at other stages appear to approach it 
very closely. 


Pleuronectes limanda, L. (Dab). — The ripe forms at St Andrews have generally been 
procured in April and May, but there is no reason to suppose that, as in other marine 
fishes, they do not overlap these limits considerably. The diameter of the egg is 
•033 inch, or about '825 mm. 

As an example we may take a series fertilised on April 30 (1885) at 2 p.m. In 
these ova the peri vitelline space is very small. At 4 p.m. the blastodisc was formed at 
one pole, and the nuclear zone covered it. At 5.30 p.m. the disc was in the four-celled 
stage, each sphere with a nucleus. Scattered granules, moreover, occurred on the 
margin. The morula-stage was reached at 10 a.m. on the second day (1st May), and 
the granular periblastic zone surrounded the disc, the nuclei being two or three deep. 
At 12.30 the disc had increased in diameter, showing finer cells in the centre, and 
larger at the margin. The periblast was broader, and nuclei could be seen under the 
margin of the disc by tilting the ovum. The disc had still further extended at 3 p.m., 
and the cells resembled large irregular nuclei embedded in a narrow protoplasmic 
envelope. On the third day the blastoderm had reached the equator, and the em- 
bryonic shield was well defined. On the fifth day (4th May) the embryo is fully 
outlined, and Kupffer's vesicle appears. The cephalic region has increased, but the 
optic enlargements are not outlined. In certain aspects indications of metameres are 
observed anterior to Kupffer's vesicle. 

On the morning of the sixth day Kupffer's vesicle had considerably diminished, 
and about sixteen metameres were indicated, and they extended almost to the head. 
At 12.30 Kupffer's vesicle had sunk into the tissues of the trunk. The metameres 
have a rounded dorsal outline. The notochord is distinct from Kupffer's vesicle 
almost to the pectoral region, and slightly indicated in front of the latter. The lenses of 
the eyes were faintly outlined at 1 p.m. 

On the seventh day (6th May) about thirty protovertebrse are clearly outlined. 
The otocysts are small, but well defined, and the cavity is ovoid and limited. The lumen 
of the mesenteron extends almost to the otocysts, and posteriorly it expands con- 
siderably, becoming attenuated, however, before ending blindly. The neurula is cleft in 
the middle line, and rises anteriorly as two bold ridges. Yellow chromatophores (round) 
are scattered over the head dorsally, and extend almost to the caudal termination. 

On the eighth day the eyes are boldly outlined, and the otocysts have expanded, the 
oval chamber having increased in length and breath. The lumen of the mesenteron 
anteriorly appears to be bifid, an arm passing towards each otocyst, but ceasing before 
reaching the eyes. The protoplasm (periblast) enveloping the yolk has formed many 
reticulations. At 12 noon the notchord shows lens-shaped cells or vacuoles — 
partially alternate in arrangment, while at 6 p.m. it is completely segmented by bold 
fissures. The yellow chromatophores are more distinct, and though irregularly dis- 
tributed may roughly be described as forming a double lateral line on each side, viz., a 
dorsal and a ventral. The surface in the cephalic region is rough from papillae on the 
dorsal and lateral regions. The pectorals are rudely outlined, and the heart appears as 


a solid transverse column, showing, however, a core when viewed on end. Active 
movements occur, so that the tail is sometimes drawn from left to right. 

On the ninth day the trunk has lengthened, and the tranverse chambers of the 
notochord are much longer, leaving narrow intervening bars of the original tissue. The 
otocysts are larger, and show two otoliths. The cavity of the mesenteron stretches from 
the otocysts to the anal region. 

The heart pulsates faintly and irregularly at intervals on the tenth day (9th May). 
The notochord is broken up into large and somewhat angular compartments. The 
pigment-spots show further development. 

By vigorous movements the embryos, measuring about -^jth of an inch in length,'"* 
emerged on the twelfth day ; but it has to be stated that in other instances, somewhat 
later in the season, and when the temperature was higher, they issued (e.g., on June 2) 
seven days after impregnation. They were carried about by the slightest surface- 
currents, gently descending head foremost and again ascending by the usual wriggling 
motion. The pigment-spots are very distinct, of a lemon-yellow colour as already 
described,t and are grouped in two lateral bands. The liver forms a pouch-like prominence 
on the anterior portion of the alimentary canal. On the second day the pigment had 
increased anteriorly, forming irregular blotches on the cephalic region. On the fourth 
day the pectorals are about twice as large as on emergence, and an anal tract is 
forming, while in many the upward flexure of the caudal region is marked. They are 
about ^jyth of an inch longer, and swim in small groups at the margin of the vessel. 

On the seventh and eighth days the chromatophores are finely stellate, and the 
eye has much black pigment. The larvae are very active, though when descending they 
often assume the reversed position. The snout is rounded and prominent ; an oral 
aperture has appeared, and the mandible slightly projects. The basal process of the 
pectorals is marked, and radial thickenings are formed on the fin. The anus is not yet 
open, and no circulation is visible though the heart beats actively. The hyoid is well 
developed, and four branchial bars are distinct. 

On the tenth day after emergence the survivors swam actively when disturbed, 
using their large pectorals like flippers, but they often lay on the bottom. The dark 
pigment of the eyes presents a greenish iridescence. The increase of the pigment over 
the surface, the opening of the anus behind the scarcely visible yolk-sac, the great 
angular development of the mandible, and the membranous opercular covering, are the 
chief changes. The stomach shows tranverse folds, but posteriorly longitudinal ruga? 
pass to the anal region. A dorsal elevation covered with papillae gives a peculiar outline 
to the head. The embryos survived only a few days longer. J 

* Mr Cunningham (op. cit., p. 100) gives the length of the newly hatched larva at 2 - 66 mm. He Joes not allude 
to the characteristic lemon-yellow coloration. 

t Page 791. 

X The stages intermediate hetween the foregoing and the succeeding are at present no doubt confused with those 
of the flounder and other forms, especially as when brought to the surface they are generally injured, or as yet have 
only been examined in spirit. 


Specimens 28 mm. in length are occasionally thrown on the west sands. They are 
distinguished from the flounders by their larger eyes and more elongated outline, even 
when the lateral line is invisible. Others 1^ inch long occurred in the trawl on June 7, 
while somewhat smaller forms were found in the stomach of a gurnard on the 20th of 

Pleuronectes cynoglossus, L. (Witch). — Another form characterised by its com- 
parative thinness, narrowness of body, great breadth of the embryonic fin, and the 
conspicuous character of the dark olive pigment, was obtained abundantly. It is distin- 
guished from the young of the long rough dab, by the translucency of the body even 
after immersion in spirit, and by the nature of the pigment, which is finely dotted 
along the ventral edge in the young dab, whereas in this form only a few (about 
five) large isolated patches, blackish in spirit-preparations, occur along the dorsal 
and ventral margins of the body (PL XVIII. fig. 7). The depth of the embryonic fin 
exceeds that of the body, and the abdomen is prominent ; the urinary vesicle being 
very visible posteriorly. A little pigment also occurs on the surface of the abodmen, 
and the marginal fin is of a faint dull yellow in life. Both sides are similarly coloured. 
The caudal region is abruptly narrowed, and the notochord proceeds straight outward 
even when fish is 8 mm. in length. The lower caudal rays are, however, cartila- 
ginous as well as those above ; all the others are membranous. The otocysts are 
large and well developed. The head and abdomen, at this time, appear dispropor- 
tionately large for a body so long and slender. By the development of the hypurals 
the usual changes are brought about in the tail, and when the fish is 12 mm. long and 
about 4'5 mm. broad (PL XVIII. fig. 8), the marginal fin is appreciably narrower, while 
the interspinous elements appear along the edge of the trunk, and inferiorly the body 
now slants from behind downward and forward, so as to embrace the gut. The ventral 
fins are not yet visible. Five pigment-patches occur along the dorsal line as before, 
besides some minute spots at the base of the tail. Inferiorly, on the right side, two 
touches are present on the abdominal edge, and one at the curve of the rectum 
superiorly ; three others lie in front of the caudal pigment-spots. On the left side 
the abdominal patches are somewhat less distinct. A little black pigment also exists 
on the tip of the jaw, at the ventral edge of its angle and on the prominent area below 
the pectorals. The right eye is apparently a little in front of the left. The embryonic 
tail remains with its superior rays, while the inferior rays (forming the main part of the 
caudal fin) are well developed. Parasitic Caligi are frequently attached to the anterior 

When 14 mm. in length (PL XVIII fig. 9), the greatest breadth of the fish, including 
fins, is about 7 mm. The left eye at this time appears, in profile, above the head, and is 
distinctly in advance of the right. The pigment-spots on the right and left sides are nearly 
the same, though those on the right, perhaps, are more distinct ; four patches occur 
along the dorsal margin and three along the ventral margin of the body. The touches 
on the abdomen are present, but somewhat altered by the growth of the tissues, and so 


with those on the ventral margin as well as the head. Pigment-specks persist at the 
base of the tail. The body is now proportionately broader. 

In specimens 2 mm. or 3 mm. longer, similar features as regards pigment and other 
points occur, but the left eye is mounting over the head, and the ventrals appear as 
minute buds, while the marginal fins of the specimens are still infested by young Caligi. 

These specimens were generally procured E. or S.E. of the Island of May, in water 
varying from 18 to 29 fathoms, the mid- water net being floated about 4 fathoms above 
the bottom. 

The earlier stages of this species have been observed by Mr J. T. Cunningham,* who 
secured the ripe adults in June by the trawl near Cumbrae in the Clyde. His oldest 
larva, however, represents a considerably younger stage than our fig. 7, PI. XVIII. , the 
latter being about 8 mm. in length, whereas Mr Cunningham's earlier form (rather more 
than two days old) measured 5*9 mm. Moreover, instead of three dark patches, there 
are four on the tail. It is satisfactory to have a fairly complete series of this species, 
which, on the eastern shores, is generally characteristic of deep water. Mr Cunningham's 
ova hatched on the 6th day, but they were under abnormal circumstances as regards 

Pleuronectes platessa, L. — The ova of the plaice (which measure "065 to '069 in.,t or 
1*65 to 17 mm.) were brought by Captain Burn, late of the 11th Hussars, on the 21st of 
April, having been fertilised two days before. The zona radiata is minutely punctured, 
and it is often peculiarly wrinkled. On the 28th the embryo is clearly outlined, and is 
conspicuous by its bright canary-yellow spots (PI. V. fig. 6). The spots do not extend 
quite to the tip of the tail, but leave a considerable terminal portion bare. In one speci- 
men a vesicle (kv), similar to Kupffer's, appeared in the mid-abdominal region, and was 
thus considerably in front of the normal position. It possessed a distinct protoplasmic 
covering. Moreover, a smaller vesicle appeared on the surface of the protoplasm of 
the larger. The heart of the young plaice presents the same features as in other pelagic 
forms, and begins to beat on the 6th day, and at 7.30 p.m. on the 28th April it pulsated 
forty times per minute. Its long tubular region lies to the left side, goes forward and 
forms a loop, turning backward just as in larval round fishes. The great breadth of the 
marginal fin is noteworthy, and it is well seen in the egg. In several examples film-like 
bands or ridges stretch across obliquely from the head of the embryo into the rest of 
the blastodermic area. As the pigment develops in the eyes some are finely iridescent, 
with a reddish-golden lustre, but in a day or two the silvery sheen surrounds the pupil. 
The eggs are hatched in nine or ten days, and the larva is conspicuous amongst its 
congeners, the flat-fishes, by its great size (PI. XVI. figs. 5, 5a). This does not imply 
that it is more readily observable, for the larvae are difficult to discern in the water. 
When about a week old the canary-yellow colour seen so distinctly posteriorly is found 
to be due to rounded corpuscles, which by transmitted light appear to be brownish, 
and more or less opaque. The marginal fin of the larva is of great breadth, though in 

* Op. cit., p. 101, pis. iii., iv., and v. t Mr Cunningham (op. cit.) gives T95 mm. 


ordinary views the body appears to be almost linear. A peculiar feature is the presence 
of minute dark pigment-specks on the ventral lobe of the marginal fin, whereas into the 
dorsal lobe (ef) only one or two of the yellowish corpuscles pass from the line of the 
body. In this early stage the otoliths are remarkably small — much less, for instance, 
than in the fluke of the same age. The larva swims actively at the surface of the 
water, and is not easily noticed except by its large iridescent eyes, which now and 
then exhibit a golden sheen. Like some other young forms already described, it floats 
head downward in the water, besides frequently boring its snout into the sand at the 
bottom of the vessel. When at rest it lies upon its side at the bottom, and if the 
background be dark the yellowish pigment is conspicuous, especially in the caudal 
region. A perceptible increase in length took place within a few days after emer- 
gence. There is so little difficulty in hatching these ova, that this species could be 
multiplied in any suitable locality which it did not already inhabit. Mr Cunningham # 
describes the yellowish spots as being in three rows on the lateral region of the embryonic 

In April large numbers of young pleuronectids at and near 12 mm. in length occur in 
St Andrews Bay. The eyes in these are generally asymmetrical, though in the smallest 
forms very slightly so. In the most advanced the left eye projects above the dorsal ridge, 
but is mainly used for vision on its own side. The blackish pigment-corpuscles are chiefly 
developed along the ventral margin of the body, though in some the sides posteriorly, 
and the posterior half of the dorsal margin, have a few specks. The terminal region of 
the notochord varies from a long dorsal filament to a mere trace beyond the hypural 
elements in the older examples. 

The foregoing may represent both the young of the plaice and the common flounder, 
the earlier post-larval stages in spirit not yet having been clearly separated. 

At the mouth of the Thames, young plaice 1^ inch and upwards abound in the nets 
of the shrimpers in October, and similar forms are met with at a later period at the 
margin of the sandy beach at St Andrews. In June and July, at the latter place, the 
smaller forms range from 2^ to 3^ inches, and these are probably the young of the 
previous season. It is a noteworthy feature in connection with this and other species, 
that tlje larger forms are characteristic of the deeper water, while the smaller, from 1 1 
inches downward, abound in sandy bays (inshore water). The mature fishes (i.e., those 
with the reproductive organs fully developed), as formerly shown, are thus mostly beyond 
the three-mile limit. 

Pleuronectes jlesus, L. — No form is better adapted for studying the development of 
pelagic Teleostean ova than this, though, as one of us has elsewhere pointed out, 
specimens in confinement seldom deposit healthy ova.t The comparatively rapid 
development of the embryo (six to seven days) is further favourable for a connected 
series of observations, The lateness of the spawning period in 1886 was also fitted to 

* Op. cit., p. 99. 

+ Vide account of appearance of retained ova, Third Annual Report of Scottish Fishery Board, 1885, p. 62. 


bring out this feature, since the temperature was thus proportionally high. Moreover,, 
as indicated in the Report of the Royal Commission on Trawling just mentioned, with, 
reference to Hi%)poglo$soides limandoides (Rough Dab) and other species, comparatively 
small specimens of both sexes are capable of successful reproduction. Thus females not 
more than 4^ inches long, and males a little larger (7-| inches), have been paired with 
perfect success. 

Ova fertilised at 4 p.m. on 1st April 1886, showed a wrinkled condition of the zona 
radiata after extrusion, but soon became smooth in outline, and the germinal cap or 
blastodisc began to be formed. In some, however, no such protoplasmic cap appeared 
for an hour or more. The two-celled stage was reached at 6 p.m., and the sixteen-celled 
stage at 9.45 p.m. The minute granules of the periblast were very evident in a profile 
view. In these ova the micropyle was generally found near the disc. Next morning 
(9 a.m. April 6) the blastoderm had made great progress, and the cells were nearly of 
equal size. At 1 p.m. it had extended almost as far as the equator. At 9 p.m. a large 
germinal cavity had appeared. On focussing down to the animal pole (the egg floating 
with the disc downward in the usual manner), a peculiar group of cells was visible, 
probably at the apex of the blastodermic cap, since the ordinary cells of the 
germ lay above them. Moreover, the two ova specially under examination presented 
certain (Brownian?) movements of the granules of the region, as if from decay, 
yet such could not have been the case, as subsequent progress proved. On the 
7th, at 9 a.m., the embryo appeared in the centre of the embryonic shield, as 
a long curved cylinder with an expanded and thickened head. It is proportionally 
longer than in round fishes, such as Gadus morrhua, G. seglejinus, and others. The 
cells of the blastoderm assume a honey-comb-like appearance — more distinct than in 
many Teleosteans. On the evening of the same day (the 7th) the optic vesicles are, 
well developed, and the tail shows a more evident enlargement in front of the tip 
than in Gadus teglefinus. Kupffer's vesicle is present, while in many examples four 
or five smaller vesicles exist on the ventral surface of the caudal enlargement. On 
the 8th April, the vesicle referred to is larger, and situated just within the blunt knob 
of the tail. It is a large clear bubble-like vesicle, bounded by slightly granular proto- 
plasm (periblast) of variable thickness. The yellowish pigment, characteristic of this 
species, now appears in the form of rounded corpuscles (PL XIX. fig. 5), which do not as 
yet send out radial processes. Occasionally one or two clear vesicles occur under the head, 
and they have the same appearance as Kupffer's structures. No other organs, except 
muscle-plates and neurochord, are visible in the trunk. On issuing from the egg the 
larvae (PI. XIX. fig. 5) float on the surface if lively, but if feeble they rest on the 
bottom in still water, i.e., in the tanks, though it is probable that this latter phenomenon 
does not occur in nature. They shoot with a wriggling motion along the surface, and 
are recognised by the beautiful yellow grains of pigment ; they appear, in fact, like minute 
clubs of transparent tissue with chrome-yellow spots. One evident patch of colour 
lies above the posterior end of the yolk, and another midway between that point and 


the tip of the tail. The pigment is also scattered along the sides of the head somewhat 
symmetrically, and produces a characteristic appearance. 

The mandible, about the eleventh day, has the form of a remarkable process in front. 
The larva differs from Agassiz's figure of Pleuronectes americanus, and shows much more 
pigment. The anus is open or nearly so. Instead of the hollow urinary vesicle behind 
the rectum a merely granular band passes downward parallel to the anal tract. 

After the yolk has been absorbed, the little flounder presents a somewhat deeper 
aspect from increase of the marginal fin, as well as the more prominent pigment on it. 
Eight touches of black pigment occur at the margin of the dorsal fin and four behind 
the vent inferiorly. The large yellowish pigment-corpuscles (about eight in number) are 
confined to the body, only a series of minute ones being distributed on the marginal fin, a 
single speck generally existing in the centre of each blackish area. The latter are larger 
ventrally than dorsally. The trunk and intestine are minutely flecked with black points. 
The anterior region of the abdomen has a few yellowish specks. Ventrally about three 
yellowish touches occur along the edge of the muscle-plates. The eyes are bluish silvery. 
A dark mass of pigment lies internally at the pectorals, probably in connection with 
the segmental ducts. The anus is at the margin of the fin. Corpuscles occur in the 
heart. The mouth is widely open, and slight movements of the mandible take place. 

As already mentioned, the ova of this species are very hardy, and the larvae 
after emergence will live for some days in a very small quantity of water, even if 

After the foregoing stages are passed, the little flounders are still pelagic, swimming 
about with eyes on both sides of the head. Like other flat fishes, however, as they get 
older they seek the lower parts of the water, though the eyes are still lateral and 
symmetrical. They are obtained by aid of the mid-water net at various stages in April, 
viz., some with the left eye still on its own side, though advanced a little and more 
prominent ; others show the eye on the edge in front of the dorsal ; while in a third 
series the left eye has gained the right side. 

In April very transparent flounders, about 12 to 14 mm. in length, occur freely 
in St Andrews Bay, and also in the sandy pools amongst the rocks. A few weeks 
later (May 24) many occur at the mouth of the lade, which pours a fresh-water stream 
into the harbour, and are caught while swimming at the surface in company with Mysis 
vulgaris, young eels, and sticklebacks. These specimens had the eye at the edge, just 
as in the case of many caught in the sandy rock-pools. Moreover, each of the examples 
referred to had a parasitic Anceus Edwardi attached externally, generally near the 
margin of the muscle-plates at the base of the dorsal fin. When the crustacean was 
removed a deep pit in the tissues of the flounder showed the point of attachment. 
Further the Anceus immediately sought a new place, and began to pierce a fresh portion 
of the skin with its sharp spine-like gnathites, and tenaciously held to the fish. After 
boring a little, a tongue-like process was thrust out, apparently for suction. The irrita- 

* Vide remarks in Report of Roy. Commiss. on Trawling, 1885, p. 363. 


tion thus produced caused the flounder to dart about with great energy.* Young 
flounders, colourless, and of glassy transparency, rapidly develop pigment in the laboratory. 

The remarkable appearance of the tail (opisthure, Ryder), with its marginal fringe 
of rays before any change takes place in the position of the eyes, recalls the condition of 
the tail in such extinct forms as Kiner's Graphiurus callopterus, in which, however, 
the vertebral column is prolonged in a straight line, instead of being bent up, and the 
ordinary caudal rays pass dorsally and ventrally from it. Kiner's form referred to, came 
from the bituminous shale of Raibl in Karmarthen.t 

The young flounders proceed a considerable distance up the fresh-water stream at a 
stage somewhat older than the foregoing. 

If the forms observed in the muddy sand of the tidal pools, and also caught in the 
mid-water net in the bay in April, are the young of the season, their growth is re- 
markably rapid, even granting a much earlier period for spawning than has been observed 
at St Andrews (April). 

During April, May, and June, very small specimens of the flounder occur at St 
Andrews in the shallow rock-pools, containing stunted Algse (Ceramium and other 
forms), with a slight coating of grey mud. From their translucency the young fishes 
are invisible, especially on the greyish silt, in which they are often partially immersed, 
and, as Alex. Agassiz noticed, the two prominent eyes alone attract attention, while the 
bodies of the fishes themselves cannot be seen. They are elongated and slender, about 
12 mm. long and 5*5 mm. in total breadth at the widest part. At this stage the true 
pleuronectid features have been assumed. They swim with the dorso-ventral line horizontal 
(the right side uppermost), and dart about with rapidity, frequently in confinement leaping 
over the margin of the vessel. They are fond of attaching themselves to the perpen- 
dicular sides of a glass vessel, as if their left (white) side had a sucker, but the adhesion 
is simply due to the muscular action of the whole surface. Both eyes are visible from 
the right side, though the left eye is more or less lateral in position, or capable of looking 
slightly downward. In company with them, plaice of the same length occur, being 
distinguishable as broader and thinner fish, with the left eye not so far to the right, 
and the ventrals as mere rudiments, while those of the flounder are well formed. 

The flounder is apparently a considerably older fish, and its left side is quite white, 
while in the plaice the pigments formerly mentioned occur. The coloration of the 
flounder varies rapidly, and though, when first captured, their anatomy is readily ob- 
served from their great translucency, yet, as indicated, a few days' exposure to an in- 
creased amount of light, from absence of shelter in the tanks of the laboratory, causes 
such a development of pigment, that they are useless as transparent objects. The blackish 
pigment-spots persisting after preservation, present a close approach to those in the young 
plaice of the same size. Thus along the dorsal body -line five pigment-spots occur, and 
four along the ventral line, almost the same number as in the former species. The general 

* The food of these flounders consists of young Gammari and similar Crustaceans, 
t Sitzungsbr. der K. Akad. Wien. Naturwiss., Bd. 53 and 54, 1866, p. 155, Taf. i. fig. 1. 


surface of the body is, however, much more generally studded with pigment-patches and 
cells, and the touches on the marginal fin are better developed. On the other hand, 
except a few minute grains along the body-line, the whole left side in some is white. 
The black pigment-spots in the American flounders, figured so deftly by Alexander 
Agassiz, show similar features, and the spots described are very generally distributed. 

The difficulties in diagnosing from size alone, are well illustrated in this species. 
Young forms, captured at different times, measured 9 mm. on the 15th April, 9 to 27 mm. 
on the 26th April, 15 mm. on the 24th May, 8 to 30 mm. on the 8th June, 10 to 18 mm. 
on the 18th June, 80 mm. on the 27th June, as well as 40 and 94 mm., while man}'' 
ranged on each side of three-quarters of an inch. In July from 22 to 32 mm. In August, 
many captured in sand-pools near the estuary of the Eden were only 12 mm. 

Rhombus maximus, Will. — The ripe ova of the turbot were procured from a female 
of 12 lbs., on the 10th July, during the trawling expeditions of 1884.* They are very 
small, only a little larger than those of the rockling, and the embryos, many of which 
were hatched from pelagic ova of the same appearance, captured by the tow-net on the 
spot, are likewise small. This seems to have been the first occasion on which ripe eggs 
of this species had been procured in this country. No oil-globule is present. 

A post-larval form procured in August in considerable numbers, both south-east of 
the Isle of May and off the Isle of May rocks, is apparently the turbot. The youngest 
example, the eyes of which are still symmetrical, measures about 6 mm., with a maximum 
breadth of about 3 mm.t The larval tail projects backward and slightly upward, 
and is still surrounded by the embryonic fin. It protrudes considerably beyond 
the inferior fin-rays developed beneath it. The head of the fish is proportionally large, — 
larger, as compared with the length of the fish, than in any other form examined. The 
mouth is large. The dorsal line is nearly straight from above the otocysts to the base 
of the tail, but the ventral line slopes rapidly downward from the tail to the anus, and 
again rises with an anterior curve to the jaw. Thus the body has a triangular outline. 
The dorsal and anal fins have rays, and are of moderate length. Papillae indicate the 
rudiments of the ventral fins. Both surfaces of the body are minutely speckled with 
black points, but the right is more uniformly marked in this way. The specks extend 
to the marginal fins, but not over them. 

The changes which follow — as seen in the next older forms — are the slight increase in 
depth and roundness of the body posteriorly, the elongation of the rays of the marginal 
fin, and the appearance of five or six touches, caused by aggregations of dots, in the 
dorsal, the ventral still remaining speckled as before. The closely approximated ventral 
fins have likewise minute black points, but the pectorals remain pale.J The right eye 
meanwhile is gradually passing upward, and the embryonic fin is rapidly disappearing. 

* Vide Report, p. 363. 

t The spawning period of the turbot in the Baltic is given as May and June, but in the North Sea, July (Mobics 
and Heincke). 

X A larval pelagic flounder of Mediterranean (Peloria riippelii, Cocco) has remarkably pedunculate pectorals, a 
feature present in many young fishes (Emery, Reale Accad. dei Lincei, Glasse di scienze fisiche, math, &c, xiv\, 1883). 


The next phase consists in the strengthening of the abdominal wall ventrally, the 
increase in the distribution of the pigment, the left side still remaining slightly speckled, 
while the right is densely coloured ; the more distinct grouping of the pigment in 
" touches" in the fins both dorsally and ventrally, and in the progress of the right eye 
towards the left. The marked notch behind the angle of the mandible, and the elevation 
of the head behind the right eye, are also noteworthy features. When the right eye 
mounts on the dorsum, the dorsal fin forms a high arch over it, and the body has con- 
siderably increased in depth in comparison with its length. A specimen about 9 mm. in 
total length has a depth of 6 mm. Besides the " touches " of pigment on the fins, a 
few minute blacks points are scattered over the left surface — the right being covered 
with minute dots almost as densely as before. 

A subsequent stage to the foregoing is shown in PI. XIX. fig. 1, but no specimen in 
our collection affords the intermediate or transition-features so as to ensure certainty by 
continuity of stages. The occurrence of the pigment-touches in the dorsal and anal fins, 
however, and their character, the general shape of the body, and the appearance of the 
head, support the probability that they are stages of the same species. None show 
traces of the spines, although the right eye has now reached the edge of the face. The 
eyes appear to be larger. Though some examples are no longer, they are somewhat 
better developed, a feature common in such fishes, certain individuals often reaching an 
advanced stage more rapidly than others which are even larger. In such an example as 
figured in PI. XIX. fig. 1, which was 9*8 mm. in total length and 7 mm. in total 
breadth, the tail measures 2*5 mm., so that the length from the snout to the base of the 
tail is nearly equal to the total breadth. The right or ventral surface is pale, with the 
exception of a few irregular black specks and streaks, while the dorsum is streaked across 
with black pigment-bands, which have a remarkably regular arrangement, the touches in 
both dorsal and ventral fins being joined by intermediate streaks, the head and abdomen 
only showing scattered points. The under surface is quite pale, and thus contrasts with 
the minutely speckled right surface of the specimen in the earlier stage. The dorsal 
and anal fins have long rays toward their posterior border, and the body of the fish 
acquires a somewhat quadrate form. The ventrals still show the pigment-streaks, and 
thus are in uniformity with the anal in a lateral view. Moreover, a characteristic 
larval cuticular spine appears at the posterior part of the head, above the opercular 
margin, and somewhat in front of a vertical line running up from the pectoral, while 
a smaller spine projects a short distance beneath. Both right and left spines are well 
marked in another example a few mm. longer, and which shows a similar coloration. 
They are probably protective spines, since they disappear as the fishes grow older. Their 
appearance on both sides, after the right eye is at the edge, indicates the possibility that, 
for some time, the fish may occasionally resume the vertical position in swimming. 
Further, the presence of a young Caligus fixed to the right side supports this view. A 
specimen, 20 mm. long, captured at the surface, shows the right eye just on the ridge, 
with the dorsal fin close to its posterior border. 


When the turbot reaches a total length of 21 mm., and when the left side has assumed 
the characteristic mottling of the adult, the spines above mentioned have disappeared 
from both sides, and the right shows minute black pigment-specks. The right eye is 
now on the left side, and the dorsal fin has advanced in front of it. The pectorals have 
considerably diminished, but the ventrals retain their proportional size. Specimens 
of these dimensions appear to be nearly a year old, and such are frequently found 
swimming at the margin of the sea. 

Our knowledge of the development of this species is meagre and very unsatisfactory. 
Thus Buckland says that the turbot spawns in early summer, Parnell states in spring, 
and the young are seen in pools and on the surface in June and July. It is asserted in 
Day's recent work* that "the young turbot would appear to swim on its edge for a 
longer period than the generality of our flat fishes ; " and it is added that a specimen an 
inch and a half in length (August) may be taken to be two months old. Day cites Mr 
Dunn to the effect that they are hatched in June or July. " For the first month they are 
quite black, and swim on edge like a ' John Doree.' Then their skin commences to mottle 
with white and brown, and their right eye begins to pass over to the left side of the 
head. Next they become white underneath, and of a light leaden colour on the upper 
surface, and during the period they remain of this shade on the back, which is until they 
have passed two months of age, they swim on the surface of the sea." Some of the 
turbot of the east coast (Scotland) at any rate spawn in July. A female on the 10th of 
that month, as already indicated, contained many ripe ova, which were of comparatively 
small size and floated buoyantly in sea-water.t Unfortunately no male could be 
procured on the occasion in question ; but many ova of precisely the same size and 
appearance were obtained on this ground in the tow-net and hatched, the larval 
fishes resembling in all the usual points those of other Pleuronectidse. They are very 
small larval fishes on emerging, and experience has shown that they could scarcely have 
the size and appearance mentioned by Day in two months. 

So far as present knowledge carries us, the young turbot of the season, hitherto 
procured at St Andrews, measure about 11 mm.J at the end of August. Others, again, 
captured in the estuary of the Eden on the 25th July, had reached 23 mm.; and one, 
from the surface, on the 20th August, 29 mm., some blackish pigment still remaining on 
the right side. In April, again, specimens about 6 inches in length occasionally occur in 
the salmon stake-nets. If these stages refer to a year's growth, the latter would seem 
to be slow, yet only very great irregularity in regard to the spawning period would 
explain such differences. 

Rhombus laevis (Brill). — No ripe brill has hitherto been seen at St Andrews, and none 
occurred during the trawling expeditions in 1 8 8 4. Raffaele considers that a pelagic ovum, 
with a large oil-globule, which he procured in February and March in the Bay of Naples, 
pertains to this species, and he is probably right. A similar ovum with a pale oil-globule 

* British Fishes. 

t Report of H.M. Trawling Commissioners, 1884, p. 263. t Total length. 


(which thus differs from that of the gurnard) has occurred in St Andrews Bay several 
times in February and March. The oil-globule did not appear to be proportionally large, 
and lay in the yolk under the lateral expansion of the embryo. The pigment in the 
latter was well developed, and mainly yellowish, though black chromatophores were also 
present; the eyes were silvery iridescent in the most advanced forms. From the 
resemblance of the contained embryo to the plaice it was at the time supposed to be 
that of the brill, and subsequent consideration of the remarks of other observers 
have strengthened this view. 

A specimen, apparently of the brill, though resembling the megrim, about 12 mm. in 
length, with a breadth of about 6 mm., was procured on August 31, 1886, off the Isle of 
May. The dorsal fin has about six dark bands at intervals, and the anal, which was 
much injured, seems to have had similar touches. The right (ventral) surface, again, 
instead of being white, is everywhere minutely dotted with black points. On comparing 
with a turbot (Rhombus maximus) of the same size, the body is seen to be narrower, the 
eyes larger, and the pectoral fins somewhat larger, while the comparative absence of 
pigment from the dorsum, and its presence, as minute dots, on the ventral (right) side 
are also diagnostic. In the former the head has less of the angular form of the turbot, 
this difference being mainly caused by the roundness of the angle of the mandible, and 
the smallness of the mouth. The specimen certainly resembles Arnoglossus ; but the 
last-named feature, the smallness of the mouth, is a point of dissimilarity. 

The subsequent stages of the brill have not yet been fully investigated, and they are 
not often met with in St Andrews Bay, not hitherto, indeed, till they reach 10 to 11 
inches, when they are common in the local trawls in September.* 

Solea vulgaris, Quensel. — On the 1st August 1884, a sole was captured 10 miles 
from land (off St Abb's Head), with ripe ova, which floated buoyantly.t No male was 
obtained, so that the development could not be followed. Mr Cunningham J gives March, 
April, and May as the spawning period of the sole, but he had overlooked this observa- 
tion. Off the eastern shores of Scotland, therefore, the period extends from May to 

In the mid-water net on the 6th July a few eggs appeared for the first time along 
with some of the gurnard, and they have since been more plentifully obtained by the 
trawl-like tow-net on the bottom towards the middle or latter end of May. Like other 
pelagic ova they are translucent, but they have the peculiarity of a more or less complete 
ring of minute oil-globules in groups, of a yellowish-white colour from refraction of the 
light, for when viewed by transmitted light they are faintly straw-coloured. When 
floating, the ring of oil-globules is superior as in other instances, the disc being inferior. 
Besides the ring mentioned, a few small groups occur here and there at other parts. 
Under a lens the egg indeed appeared to be flecked with yellowish-white pigment. In 

* Vide Trawling Report, pp. 358 and 361. 

t Report of the Trawling Commissioners, p. 363. 

X Jour. Mar. Biol. Assoc, N.S. i. p. 18, where an excellent account, with figures, of the early stages is given. 


diameter the ovum measures # 045 inch. The large oil-globules have a diameter of '0015 
inch, while the smaller measure "0004 inch in diameter. The capsule, in a few slightly 
undulated, is somewhat thick and tough, so that considerable force is necessary to rupture 
it. The zona is very distinctly punctate, even more so than in that of the plaice 
(PI. I. fig. 20). In one example the surface of the zona was covered with flattened 
papillae, giving it a scabrous aspect (PI. X. fig. 7). In the early condition of the blasto- 
derm the border of the yolk under it presented a few large vesicles (PL XXII. fig. l), 
which projected beyond the edge of the periblast, and at a later stage this vesicular 
condition extended round the greater part of the yolk, except just at the tail of the 
embryo. Moreover, pigment rapidly develops over the surface of the yolk as well as 
on the head of the embryo, and it has a dull whitish or faintly yellowish hue, in marked 
contrast to the yellow tint of the gurnard. 

When the embryo is fairly formed' (PI. II. fig. 11), the groups of oil-globules change 
their position, most occurring along the ventral surface of the embryo, as in the egg of a 
Solea (?) described by Raffaele (No. 125a, p. 42, Taf. 1, figs. 33 and 34). # 

The oil-globules in this egg comport themselves differently from the single globule in 
other eggs, e.g., of the gurnard. They do not move freely, so far as observed, at any 
period of development, but retain their positions during the motions of the ovum. Their 
relation to the periblast must therefore differ materially from that in the gurnard already 
described. Raffaele considers they are in the cortical protoplasm, which divides the 
vitelline segments, and move with the latter. They certainly advance with the rim, 
but their subsequent arrangement under the developing embryo is a remarkable feature, 
indicating, indeed, the probability that something like a streaming of the protoplasm of 
the periblast takes place about the period of the closure of the blastopore, so as to carry 
the globules under the developing embryo. 

While in the living egg the foregoing is the condition so far as can be observed, it is 
otherwise in the dead egg after the lapse of a day or two. In a dead egg at the morula- 
stage, the oil-globules (now somewhat larger and of a dull yellowish colour) had grouped 
themselves at the upper pole, the disc being at the lower. When the disc was placed 
uppermost the oil-globules moved up to it at first apparently on the surface of the yolk, 
but a more minute examination showed that they also moved through the yolk. It is 
clear, therefore, that a change had occurred in the protoplasmic investment of the yolk so 
as to release the oil-globules, which to some extent had coalesced, and permit them to 
pass through it. 

The eggs develop with moderate rapidity, so that those with the rim about a third 
over, and which presented segments in the periblast under the blastoderm (forming the 
vesicular condition), hatched on the fourth day thereafter. The larval sole is a character- 
istic form (PI. XVII. fig. 13), the entire body, yolk-sac, and marginal fin being minutely 

* Mr Cunningham, in Ms recent paper, describes the oil-globules as aggregated on each side of the embryo, though 
there are a few groups at other parts of the surface of the yolk. He figures other stages than those given in this 
paper, and shows the vesicular condition at a different period from that in our fig. 1, PI. XXII. 

VOL. XXXV. PART III. (NO. 19). 6 Q 


speckled with opaque yellowish-white pigment. This pigment is arranged in interrupted 
touches on the body and marginal fin (dorsal and ventral), behind the yolk-sac, so that 
the pleuronectid character is early indicated. Moreover, the presence of pigment at 
the extreme margin of the fin, both dorsally and ventrally, gives great apparent depth to 
the body of the fish. The yolk-sac is comparatively large and globular, sustaining the 
larval fish readily in the water, either as in ordinary cases (sac uppermost), or suspended 
from it tail downward. Occasionally it remains in a vertical position with the head 
downward. The large and rounded condition of the yolk-sac causes the active little fish 
to roll over during progression, so that it often advances in a screw-like fashion. While 
in lateral view the yolk-sac is somewhat ovoid, it is quite circular when seen either from 
the front or the rear (PI. XXIII. fig. 10). The same condition probably causes the larva 
to make frequent gyrations. It would appear to be one of the most restless of the group, 
seldom remaining quiescent under examination more than a few seconds. It is not quite 
3 mm. in length. The oil-globules form two main groups, one series running from the 
heart obliquely backward to the region of the pectoral fin, the other at the posterior 
part of the yolk, and extending ventrally along the posterior border (see fig. 13, PI. XVIL). 
They slightly vary in different specimens. One or two isolated groups also occasionally 
occur along the ventral border. All retain their periblastic position. No pigment other 
than the superficial chromatophores exists in the eyes. 

The vesicular condition of the yolk is not readily seen after hatching, though it can 
be made out by manipulation of the light, or in favourable positions. The vesicles 
appear to be flattened out at the margin of the yolk. In a specimen of the first day, 
peculiar vesicles, having a faintly pinkish hue like those of the blastodisc of the haddock, 
were visible on looking down on the yolk-sac of the larval fish floating head down- 
ward (PI. XXIII. fig. 10). They were grouped in the neighbourhood of the posterior 
oil-globules, and occurred nowhere else in the yolk. They differed in appearance 
from the ordinary vesicles at the border of the yolk, and resembled peculiarly modified 
protoplasm. Their globular condition was distinctly visible during the motions of the 
larva, and they were situated in the transparent yolk within the oil-globules. One of the 
vesicles presented a series of minute granules in its interior. They were observed subse- 
quently in various specimens. 

One example presented a vesicular process over the brain, so that it had a hooded . 
aspect, but this enlargement appeared to be abnormal. 

On the second day the yolk has considerably diminished, and the posterior border 
carries the groups of oil-globules forward with it, leaving a larger space between it and 
the vent, while the pericardial chamber has become distinct in front. Minute pigment- 
specks now appear in the eyes. The peripheral segments of the yolk are still indicated. 

On the fourth day the yolk has still further shrunk. The cavity of the mouth is 
formed, though no external aperture yet exists. The vent has not yet opened, indeed 
the gut terminates a little within the margin of the fin. The clear vesicles observed in 
the yolk of the former specimen were still visible, and one had a minute globule of oil in it. 


A feature of interest in several was the remarkable size of the optic lobes, which projected 
dorsally so as to give the head a " hooded " aspect, as in the condition before mentioned. 
The agility of the little larva is characteristic. 

Three days later the activity of the larval fish had become even more marked, and it 
seemed in a state of perpetual movement, the only interval being for a second or two 
after a long course through the vessel. This almost ceaseless movement is probably 
connected with respiration, the now widely open mouth being driven against the water 
which thus rushes into it. The pectorals vibrate like those of Hippocampus (a re- 
semblance the more appropriate from the dermal process on the vertex), and the tail 
appears to move as rapidly. The larval soles chiefly kept the bottom of the vessel at 
this stage, swimming obliquely with the head directed downward, as if boring into the 
bottom or sides. Occasionally, however, a swift dart was made right across the vessel, 
or a shorter one as if capturing prey. The mandible moves rapidly as in respiration. 
The yolk has now diminished to a small mass anteriorly — with the groups of oil-globules 
crowded together, while the posterior region of the abdomen is occupied by the viscera. 
This forward progress of the yolk is interesting, for while different conditions occur in 
different groups, one of the most common is the absorption of the anterior region, 
and the consequent presence of the diminished yolk posteriorly. Another feature 
of note is the occurrence of a prominent fold along the ventral margin of the abdomen. 
The pigment seems in some to be more ochreous, and to have less of the dull yellowish- 
white (like Tripoli powder) so characteristic of the early condition. Along the dorsal 
margin of the muscle-plates are a series of pigment-patches, which appear to be more 
numerous than in the example of the post-larval stage elsewhere described,* but variations 
may occur in this respect. 

As the larval sole gets a little older, for instance two days subsequent to the preceding 
stage, the pigment becomes more distinctly ochreous, and the yellow chromatophores 
along the dorsal edge of the muscle-plates show signs of increase. Moreover, the pigment- 
spot on the occiput so characteristic of the subsequent stage is outlined. Eight distinct 
pigment-patches occur behind the former, one of the posterior (seventh from the occipital) 
being larger and almost meeting that from the inferior edge. The character of the head 
is as peculiar as in the previous stage, and the eyes are directed more or less forward 
.(forward and outward), so that the active little fish can readily see in front. The yolk 
has now shrunk to a small mass under the liver — in front of the gall-bladder, and is not 
easily distinguished. The change from the buff or stone-coloured, or even the dull 
yellowish-white, of the early stage, to the ochreous tint of the present one is a feature of 
interest. Moreover, one of the most marked changes is the disappearance of the yellowish- 
white pigment from the edge of the marginal fin, so conspicuous in the early larva, and 
which renders it so easily observed in a glass vessel. The speckled condition may be 
associated with the more helpless stage, when, perhaps, it frequently rests on the 

* Vide Ann. Nat. Hist, Dec. 1888, p. 469, and Seventh Annual Report, Fishery Board for Scotland, 1889, where a 
coloured figure is given. ■ : 


bottom, but this is conjectural. At any rate, the border of the marginal fin, at this 
and the subsequent stage elsewhere figured, is so translucent as to be generally in- 
visible, only the pigment-touches arising from the border of the muscle-plates being seen. 
The other parts of the head and body, as well as the ventral surface of the abdomen, are 
speckled with ochreous and black pigments. It would seem that the pale buff or yellowish- 
white pigment of the early larva is transitory, for by and by the ochre-yellow, beginning 
at first as very minute points over the head and body, gradually spreads and supersedes 
the yellowish-white, which disappears. The differentiation between the two is clearly 
seen at certain stages, the yellow being characteristic of the body, the pale buff or whitish 
of the marginal fin. The pectorals have their fan-like distal regions directed forward, so 
that the larva seems to row itself onward by their rapid motion. The basal parts of the 
pectorals are also invaded by the yellowish pigment. The eyes are silvery with black 
pupils, and a dark arch occurs superiorly. The great depth of the head and the prominent 
ridge over the optic lobes are characteristic. Moreover, the skin-fold along the median- 
line of the abdomen next day was marked by a central hiatus, so that it formed two 
portions. Further, the anterior one in a day or two became broad and almost vesicular. 

Zeugopterus jpunctatus. — A female example distended with ova was obtained in a pool 
near the laboratory, on the 16th May. Most of the ova were unripe, but here and there a 
translucent egg (PL I. fig. 6) occurred, especially anteriorly. They had a diameter of '042, 
that of the conspicuous oil-globule being "008. Though, in all probability, not so large as 
perfectly mature eggs discharged into the sea, the size is approximative. As might be 
expected from the comparative scarcity of the adult off the eastern shores, the pelagic ova 
are extremely rare in tow-nets ; indeed, so far as known, none have been met with. 

A post-larval example, 9 mm. long., was captured by the mid- water net at 25 fathoms, 
south-east of the Island of May, 30th August 1886, though unfortunately it was consider- 
ably injured. It is easily distinguished from the turbot of the same size by the much 
larger bright silvery eyes, and by the outline of the body. The right eye is prominent 
on the edge and its axis is directed laterally. The abdomen appeared to be prominent. 
It is an older fish than the turbot of the same length. 

The size and prominence of the eyes in the latter stage is noteworthy, for when the 
fish reaches the length of 3^ inches they are proportionally less, and moreover they are 
deeply sunken. 

Unknown Larval Pleuronectid ? (A). — When using the tow-net on July 9, 1884, on a 
trawling expedition 47 miles east by south of the Island of May, and over very rich ground, 
a larval fish about 3 mm. was obtained by one of us. At first sight (after preservation) 
it resembled a heteropod, for a cylindrical process projected from the anterior end, and the 
position of the yolk-sac and other features increased the likeness. The anterior process, 
however, is a hernia cerebri, and it must be remembered that the optic lobes in the Pleuro- 
nectids are prominent. The mouth is indicated by a faint slit. The marginal fin is well 
marked, extending from the front of the head to the tail, then forward to the anus. Here 
it splits, a fold running along each side of the yolk-sac to the posterior part of the mandible. 


No form hitherto examined shows this double frill so well, a feature probably connected 
with the peculiar condition of the ventral surface of the abdomen. In relation to the 
latter, we have immediately below the small and vertically elongated pectorals a spherical 
body, the liver, then a smaller mass (gut ?), and lastly the large ovoid swelling of the 
yolk, which is closely applied to the gut above and to the rectum behind. The latter is 
well marked, and appears to open by an anus at the tip. 

Unfortunately the preservation of this specimen was defective and the sections unsatis- 
factory, but one feature of note was observable, viz., the fact that the yolk contained a 
large oil-globule surrounded by a belt of protoplasm in which were a series of small 
oil-globules, which thus formed a ring round the larger central one. The lateral fold on 
each side of the yolk showed epiblast outside a core of intruding mesoblast. 

Certain features in this form approach those of the larval Arnoglossi, described by Dr 
Raffaele (125a, pp. 49-55). Zeugopterus, Rhombus Icevis, and probably other Pleuro- 
nectids, however, also have an oil-globule in the egg. 

Ovum of Pleuronectid (B) — with large peri vitelline space. — This large ovum, 
frequently met with in the trawling expeditions of 1884, and every year since, 
is characterised by its large peri vitelline space, in which the yolk with the early blastoderm 
floats freely like a globule. At a later stage (PL XIII. fig. 3) the yolk keeps the 
upper arch of the egg with the embryo curved beneath. The zona radiata is com- 
paratively thin, and it is sometimes difficult to obtain a clear view of the minute 
punctures (PI. X. fig. 8). It is, however, not devoid of toughness. The contained 
embryo shows chrome-yellow and blackish chromatophores, the former extending nearly 
to the tip of the notochord. The newly hatched larval fish has been figured and described 
elsewhere,* so that it is only necessary to mention the later stages. The larval fish 
during the absorption of the yolk often shows prominent processes projecting from the 
surface of the yolk into the anterior space. When the yolk has been absorbed the fish 
presents three distinct yellowish bars behind the vent (PI. XVIII. fig. 2), another at the 
latter (vent), and a line along the dorsum of the intestine, besides various touches of the 
same on the head and elsewhere. Stellate black pigment-corpuscles occur along with the 
yellow, and in the early condition are present over the yolk. The eyes soon assume a 
silvery aspect. The larval fish is active and comparatively large, resembling in certain 
respects the plaice. It is probably a pleuronectid. 

Mr Cunningham describes the same egg before hatching.t It is not uncommon both 
in St Andrews Bay and in the open sea beyond. 

Unknown Ovum (C). — Besides the foregoing, a small undetermined ovum occurred in 
the mid-water net in April, and probably belongs to the same group (Pleuronectidse). 
The contained embryo is comparatively large and fills up the capsule almost completely. 
The larva issuing from this egg is represented in fig. 1, PI. XVIII., the dull brownish- 
yellow pigment being characteristic. Moreover, the mouth of the embryo is open at the 
period of hatching — as in the plaice. 

* Vide Seventh Annual Report, Fishery Board for Scotland, 1889. t Op. cit., p. 105, pi. vii. fig. 2. 


Clupea harengus, L., and remarks on Clupeoids. — The youngest stages (A) of the 
herring were those hatched in the laboratory, 6th March 1885, and they measured 7 mm.* 
They are distinguished by their elongation, by the situation of the anus, which lies behind 
the commencement of the posterior sixth of the body, by the vesicular yolk, and by the 
ovoid condition of the yolk-sac. The mouth is conspicuous in some, in others it is not 
visible, therefore it is probable that there is diversity in regard to the degree of develop- 
ment at the period of hatching, as indeed the variable length shows. The pectorals stand 
at a slight angle to the body. The marginal fin is dilated in the caudal region. These 
specimens seem to be larger than Dr Meyer's Baltic herring, which were only from 5 '2 
to 5*3 mm. in length, and the same length is given by Kupffer. 

Considerable progress had been made on the second day (stage B), for good examples 
measure 8 mm. , and the body is less filmy. The yolk-sac is elliptical rather than ovoid, with 
the marginal fin carried forward on its surface posteriorly. A slight opacity occurs above 
and below the tip of the notochord. An opaque internal process also appears some distance 
in front of the anus. The mouth is a mere fissure, for the mandible is not much developed. 
A faint black pigment-line runs along the ventral border from the yolk-sac to the anus. 

The next stage (C) is represented by examples caught in the mid- water net at 4 fathoms 
off the East Rocks, 29th March 1887. These Clupeoids are now about 10 mm. in length. 
The general outline of the fish is still much elongated, the snout is blunt, the eyes 
large and prominent, with a silvery lustre and a black arch of pigment superiorly. The 
mandible projects considerably in front of the snout. The otocysts are so large and pro- 
minent that the body appears to come off abruptly from the anterior region. The pectoral 
fins are similar to those in the foregoing stage, but the marginal fin has disappeared from 
the body, and a small elevation occurs on the dorsum (noticed even in examples two days 
old), some distance in front of the anus. The caudal arises about midway between the 
anus and the tip of the notochord (which is quite straight). Its outline is spathulate, 
and there are many embryonic rays. The ventral pigment now forms a dotted line on 
each side, between the pectoral region and the anus, and some specks also appear on the 
ventral border of the notochordal region at the tip of the tail. The anus is at the com- 
mencement of the posterior sixth of the body. 

This form is evidently considerably older than the second, as the advances in the 
head, the hyoidean, branchial, and mandibular regions show. The branchial arches project 
freely ventrally. It is probable that it is at least a week or two older, a period which 
would correspond with the deposition in March, and those captured appeared to be about 
the same age, and were in great abundance amongst Sagittse, Medusae, Zoese, exuviae of 
Balani and other forms. Meyer observes that free herrino; at the age of a month are 
17 to 18 mm. in length, so that the foregoing, according to this author, would be consider- 
ably less than a month old. It has to be borne in mind, however, that there is great 
variation in the growth of fishes. 

* The form of these was much more elegant than the larval herring represented by Mr Cunningham, Trans. Roy. 
Soc. Edin., vol. xxxiii. pi. i. fig. 3. 


About a fortnight later, viz., on the 14th April, young Clupeoids (stage D) were 
procured off the Pier Rocks by the mid-water net at 4 fathoms, along with a few young- 
sand-eels, which are distinguished in spirit by their greater opacity and the larger pigment- 
spots forming an interrupted ventral line, as well as by the more or less median position 
of the anus. They are now from 12 to 15 mm. in length, and show the interrupted line of 
black pigment-spots from the pectorals to the middle of the body, after which the spots 
are so closely approximated that they seem to form one line to the anus, which has a 
speck or two externally on each side. These pigment-touches are all elongated antero- 
posteriorly, those behind the middle being linear. A few specks also appear on the 
ventral part of the caudal, next the notochord, and sometimes above the latter in those 
most advanced. The somewhat thick notochord passes straight backward, and the 
general outline remains spathulate. The embryonic fin-rays are still present, but the 
ventral region of the tail shows considerable opacity from the development of the 
hypural elements. A delicate narrow marginal fin is continued forward from the tail to 
the vent, and from the front of the latter a thin border runs ventrally almost to the 
pectoral region. Dorsally in the region of the process formerly noticed (i.e., above a 
vertical line in front of the anus), a permanent dorsal fin is developing, its posterior 
border being somewhat abruptly sloped, while its anterior runs into a thin marginal fin 
which proceeds some distance forward. The base of this fin is opaque. The upper jaw 
has increased in length, but the mandible is only slightly longer. The mouth forms a 
large transverse slit. The brain and spinal cord are clearly seen anteriorly, and the 
otocysts are still large. The branchiae communicate freely with the water. 

At this stage the fishes are probably not less than a month old. 

On the 28th April (two weeks subsequently) most have reached the length of 16 mm. 
(stage E), and the depth of the body has notably increased. The dorsal fin is larger, and 
so is the caudal, while the ventral opacity in the latter is also greater. The ventral and 
caudal pigment is more distinct, and most specimens present a median streak of pigment 
in front of the pectorals. The opercular fold is now growing over the branchiae, which 
do not yet show papillae. Viewed from above, the snout is broadly spathulate ; and the 
alimentary canal is generally empty. Four days later all the structural features just 
mentioned were better marked, and the notochord showed a tendency to bend upward 
at the tip, but there was no increase in length. 

A notable enlargement was observed on the 16th May (stage F), the length being 
now 20 mm., and the depth in the median region of the body was much greater, the part 
immediately behind the pectorals having, however, a less depth than the succeeding, but 
it was thicker transversely, so that there was less abruptness between the head and the 
body. The pigment- touches along the ventral edge are much larger, still, however, 
preserving their elongated shape and disposition — that is, arranged as an anterior series 
of larger and a posterior of smaller specks terminating at the anus. The latter is 
situated at this stage about the commencement of the posterior seventh of the body. 
The snout retains its spathulate outline, the pectorals are large, and the dorsal shows 


fin-rays ; the position of the latter fin, moreover, is unaltered. Behind the anus the fine 
rays of the anal fin are visible for some distance. A marked thickening, forming a rounded 
boss anteriorly, now exists under the tip of the notochord, which is slightly bent up. 
True caudal rays occur from the latter thickened region to the tip of the notochord, the 
embryonic fin completing the margin dorsally and in front ventrally. The direction of 
the inner border of this hypural thickening is from above downward and forward, the 
pigment marking it externally. The tail is thus being pushed upward. This stage is 
probably between two and three months old. 

The next stage (G) at present available is illustrated by a specimen procured on 
1st July, and measuring 27-28 mm., or about 1^ inch. This has now assumed most 
of the characters of the adult. Thus the head has become more elongated and com- 
pressed laterally, and the upward bend of the mandible is marked. The depth of 
the body has much increased, so that the fish appears to be shorter. The dorsal fin 
is shorter, and has an elaborate muscular ridge at its base. It stretches from a line 
over the tips of the ventrals to the first third of the anal. No part of it extends 
in front of the ventrals — that is, it does not reach their anterior ends. A row of black 
pigment-spots runs on each side of the dorsum backward to the dorsal edge of the 
caudal. The anal begins at the posterior fourth of the body, instead of the posterior 
sixth or seventh, as in the earlier stages, and such is therefore a distinctive feature. 
The pelvic fins arise from a point rather in front of the middle of the body, and thus 
their position differs from that in the adult. The pectorals are still proportionally 
large, with a fan-shaped basal region and expanded rays. The caudal is deeply 
bilobed. When viewed from the dorsum the head smoothly glides into the body — from 
the great increase in the thickness of the latter. The caudal is homocercal, the basal 
(or hypural) region having a double crescent, and the pigment has increased in this 
and the neighbouring part of the base. 

This form may fairly be considered as representing at this period the direct continua- 
tion of the stages formerly mentioned, though perhaps it is an advanced one of the series. 

The second series of the season commenced with two examples procured on the 30th 
August. They nearly correspond with stage D of date 1 4th April. 

On the 24th September, again, three stages occur, viz., those corresponding to stage 
E in spring; secondly, one, though only measuring 14 mm. in length, showing a further 
stage of development than stage F of 16th May (and possibly a sprat), for the hypural 
elements form a nearly straight vertical edge posteriorly, and the tip of the notochord 
projects from the upper angle ; and thirdly, one a millimetre or two shorter than stage 
F, but somewhat more advanced than the previous (stage 2) in regard to depth of body, 
firmness of muscle, size of dorsal fin, and especially in the condition of the caudal, which 
has a straight vertical edge, with the permanent dorsal rays developing over the tip of 
the notochord. 

A considerable margin must thus be given in regard to the spawning period. 

On the 1st of October, again, one corresponding nearly to stage 2 of 24th September 


was procured, the posterior edge of the hypurals not being quite vertical ; while the 
upward bend of the notochord is in the form of a gentle slope. 

Various stages were obtained on the 11th October in the same haul of the net, the 
earliest being like those last mentioned (1st October). The most advanced (about 19 or 
20 mm. in length) had well-marked dorsal and anal fins, vertical hypurals, and just a 
trace of a notochordal spike at the dorsal edge, and therefore intermediate between F 
and G (1st July). 

These remarks would tend to indicate that, at least, two spawning periods, as already 
known in regard to ova, occur in the neighbourhood. 

Some whitebait procured in the Thames in June measured from 38 to 40 mm., and 
presented most of the adult characters. These perhaps represent the young of a late 
autumnal brood, though, judging from those procured in St Andrews Bay in July, a close 
approach must be made by the winter broods, especially in the warmer southern waters. 
Meyer's statistics would further corroborate this view. Similar Clupeoids abound in St 
Andrews Bay in March, and these may fairly be held to be the young of the previous 
season. According to Meyer's statistics, such would be about 5 months old, but probably 
they were from the ova of August, a period of seven months. 

The gradual change in the position of the anus, by the elongation of the region 
between it and the tail, is noteworthy, as also is the relative position of the fins in the 
young and in the adult. The latter, which has been called the migration of the dorsal 
forward, was pointed out clearly by Sundevall and various subsequent waiters, and 
appears to be characteristic of the Clupeidse. The recent remarks of F. Raffaele 
(No. 125a) on this subject are of much interest. 

Clupea sprattus, L. — About the beginning of May numerous transparent ova having 
a delicately reticulated yolk and somewhat thin zona radiata occurred in the bottom 
trawl-like tow-net. They appear to be the same as Hensen first found in the Baltic, and 
Cunningham obtained in the Firth of Forth west of Inchkeith, and which are described 
and figured by him. Hensen truly indicates the pelagic egg of the sprat as having a thin 
and transparent zona ; while the larval form, he states, is distinguished from that of the 
herring by a slight flexure of the intestine.* Many are not quite round, their long 
diameter being '044 inch, and their short diameter # 039. The reticulations of the yolk 
(PL I. fig. 5) are very fine, and much less distinct than indicated by Mr Cunningham, the 
margins of the sphere in an ordinary view presenting a confused series of lines. These 
eggs occur in very considerable numbers, and are evidently those of an abundant species. 
They are easily recognised from ova which resemble them in size by their translucency 
and the colourless embryo. They develop very quickly, and the larva soon escapes as a 
translucent form about 3*6 mm. in length, and, as Mr Cunningham says, is at first devoid 
of pigment. It is a characteristic Clupeoid (PI. II. fig. 13), with the anus situated 
posteriorly. The yolk has the same kind of reticulation as described above, and it is 
comparatively large. Well-marked sense-organs are present on the sides, the last pair 

* Funfter Bericht der Komission z. wiss. u. d. deutschen Meere, 1887, p. 40. 
VOL. XXXV. PART III. (NO. 19). 6 R 


(opposite the anus) being larger than the others. Five pairs occur behind the yolk-sac, 
while a sixth exists in front of its posterior border. These organs are not opposite each 
other, but the left is a little in advance of the right. The marginal fin is not deep, and 
extends a short distance on the yolk. Very fine cells are visible on its surface. 

The young fishes are somewhat delicate in confinement, the oldest example reared 
in the laboratory being represented in Plate II. fig. 13a — about nine or ten days after 
hatching. The yolk-sac has now shrunk considerably, and the snout projects forward as 
a blunt process. The surface of the yolk-sac anteriorly in one example is minutely 
papillose, but this is probably an abnormality. The pectoral fin is well developed, and 
the eye is slightly silvery. The mere change of these young fishes from a deeper to a 
shallower vessel suffices to cause distress, with speedy opacity and death. 

Ammodytes tobianus, L. — Young sand-eels were found during the Trawling Expedi- 
tions in great numbers about the middle of April,* and they are similarly met with 
annually in St Andrews Bay, generally at a depth of 4 fathoms. 

The youngest form associated with the sand-eel was procured in the mid- water net on 
the 29th March, and measured 6 or 7 mm. in length. The body is slender and elongated, 
while the head is large and bluntly rounded in front. The mandible projects considerably 
beyond the premaxillary region when the mouth is widely open. The pigment of the 
eyes (in spirit) is black, and scarcely a trace of the silvery sheen is noticeable. The eyes 
closely abut on the front margin of the snout. The notochord passes straight backward 
in the centre of the tail, which has only the fine and symmetrically arranged embryonic 
fin-rays. The delicate marginal fin had been injured, and only a remnant existed in front 
dorsally. The pectorals are largely developed. The anus opens about the end of the 
middle third of the body. Black pigment-specks are distributed along the ventral 
surface, viz., a single line from the pectorals a short distance backward, then a double 
line (on each side of the gut) to the anus. Behind the latter a very closely dotted line 
extends to the base of the tail. 

A large number of larval forms similar to the foregoing, though somewhat longer 
(9 to 11 mm.), abounded in St Andrews Bay about the beginning of April, but their 
identity is at present uncertain. 

What appears to be the next older stage (between 8 and 9 mm.) was captured on the 
14th April. The marginal fin (which occurs all round) shows no differentiation, but the 
increase of the hypural elements and the true fin-rays inferiorly cause a slight upward 
bend of the tip of the notochord. A single dotted line of pigment passes from the 
pectorals to the tip of the tail, and a series of large pigment-corpuscles exists on each side 
of the alimentary canal in the middle third. The eyes now show a slightly silvery sheen. 
The mandible is still prominent. Cartilaginous rays occur in those parts of the dorsal and 
anal fins behind the vent. The pectorals are very large, much larger proportionally 

* Mobius and Heincke give the spawning season of A. lanceolatus, according to Bloch, in May, and mention that 
Malm found a female with enlarged ova in June. A. tobianus, again, is said to spawn in summer (i.e., from May to 


than in the adult, the basal region being massive and muscular, while the distal forms 
a broad fan-shaped fin still having embryonic rays. It would appear that the relative 
sizes of the basal and the distal regions of this fin vary according to the different stages 
of the young fishes, the basal being especially large in the early larval condition, and 
gradually diminishing as the older stages are reached. These form most efficient organs 
during the purely pelagic life of such fishes. The branchial arches show small rounded 
papillae (representing the branchial lamellae). The otocysts are large and prominent. 

This form seems to vary considerably in length in the subsequent stages, thus, e.g., 
on the 28th April, some, though further advanced in general structure, were shorter than 
in the earlier condition. The snout shows less of the previous disproportion — the pre- 
maxillary region having grown outward so as to project almost as much as the mandible. 
The tail forms a symmetrical fan-shaped organ, the base presenting a straight vertical 
line (hypural), while the upper edge is pointed, from the tip of the notochord. The 
marginal fin is prominent from the vent inferiorly, and somewhat in front of this dorsally, 
rising a little in each case in the middle, and diminishing toward the tail, which it 
joins. Permanent rays occur in both, the anterior and posterior ends, however, being 
devoid of them. The black pigment forms in front of the anus two lateral rows of large 
spots, and a median more continuous series as far as the anus ; while behind the latter a 
row of smaller specks exists on each side of the median line. Just in front of the 
pectorals a black pigment-bar occurs on each side. 

At 12 mm. in length (also in April) the body has considerably increased in depth, 
while the tail-fin is now more elongated, and presents a median notch. The fin-rays in the 
dorsal extend distinctly forward to a line running upward from the anus, and less clearly 
for some distance in front of this. In the anal fin the rays reach the anus. These fins 
are at this stage distinctly separated from the caudal, and the base of the latter has a 
double crescent at the edge of the hypurals. The oblique bars of pigment in front of the 
pectorals, and the black pigment-spots along the ventral line are well marked, especially 
in front of the anus. The anal has now a double row of minute black pigment-specks 
at its base, a feature apparently coincident with the development of the rays. The 
mandible slightly projects beyond the premaxillse. On 28th April they ranged from 
9 to 14 mm. in length. Like other food-fishes, this species is subject to the attacks of 
parasitic young Caligi. 

The next stage in the collection was procured in the mid- water net on the 5 th May 
1887, at the depth of 4 fathoms on 6 fathom ground, and they reached from 17 to 18 
mm. in length, though some were less. The shape and arrangement of the pigment-spots 
generally agree with those mentioned in the last stage. The tail still shows two hypural 
crescents — with a dotted line of pigment running from the dorsal to the ventral edge. 
The rays of the dorsal fin, though short, can be traced forward to a point midway 
between the anus and the pectorals. The eyes are still proportionally large, and the 
mandible projects in front of the premaxillse. The branchiae have simple papillae, with 
at most traces of crenations at the sides, and the gill-rakers are developing. The same 


stage was found on the 11th October 1886, so that either the spawning period is pro- 
longed, or two spawning periods occur. 

On the 7th May 1884, a form still further advanced was caught in the tow-net in 
Aberdeen Bay, its total length being 27 mm. This would therefore correspond in some 
respects with the progressive growth of the foregoing, though the irregularity in this 
respect of marine as of fresh- water forms renders great caution necessary. The dorsal fin 
has now nearly reached a vertical line from the tip of the pectorals, but not quite, and it 
shows short fin-rays anteriorly. From the vent backward both dorsal and ventral fins 
have long rays — much longer than in the previous stage. The hypural crescents are 
crossed by the caudal fin-rays. The snout has grown still further in front of the eyes, 
and the head more closely resembles that in the adult. The branchial processes now 
present well-marked papillae, and the gill- rakers are longer than in the previous stage. 
The former come off nearly at right angles, but the rakers slant differently. A 
membranous marginal fin appears ventrally from the anus about a third along the 
abdomen, and less distinctly somewhat further forward. 

While the larval forms occur in March, it is necessary to state that others much 
larger are captured by the mid- water net at the same time. Thus, on the 29th or 30th 
March, three were obtained — 18, 22, and 25 mm. in length respectively — such being in 
all probability the young of the previous season. Day concludes that they spawn in the 
autumn and winter months. He found in A. tobianus the " milt and roe " advanced in 
August and September, while Thompson states they deposit spawn at the end of July, 
but in some places they do so during the winter. Couch, again, considers the end of 
December the most common period, so that Day is of opinion that they continue 
spawning in the sand through the last few months of the year. 

Again, an example, 33 mm. in length, was caught in the mid- water net on the 10th 
July 1887. Finely ramose pigment-corpuscles now stud the dorsum for a short distance 
behind the pectorals to the base of the tail, and the head and opercular regions have also 
an increased number. The tip of the snout is likewise similarly pigmented. The fleshy 
base of the pectorals has become much less in proportion to the distal region, from the 
increase in the length of the fin-rays and the consequent alteration in the shape of the 
organ. The pectorals, indeed, seem to attain their maximum at this stage. The 
elongation of the snout and the prominence of the tip of the mandible are also features 
of note. In regard to the pigment of this and other stages, it may be observed that 
considerable differences exist, according to the condition of the corpuscles. When 
contracted they form mere points — leaving the dorsum comparatively pale, whereas in 
expansion they constitute finely ramose pigment-areas with a central black point. 

At 45 mm. in length (August) the snout still further elongates, and the hollow in the 
turbinal region disappears. From the increase in length and bulk of the body the pectoral 
fins do not have the same proportional size as formerly. The other characters are those 
of the adult. 

Unknown Larval Fish (D). — An unknown larval fish (PI. VIII. fig. 1) was pro- 


cured in April, with the anus about the centre of the body, and a small mass of yolk 
containing a considerable oil-globule towards the posterior part of the abdominal cavity. 
It is less elongate than a Clupeoid. The gut passed backward for a distance somewhat 
longer than that between the snout and the abdominal wall behind the yolk-sac, and 
terminated about the middle of the marginal fin. A line of black chromatophores ran 
along the abdominal wall ventrally, and another (subnotochordal) was continued over 
the gut. The notochord was unicolumnar. In certain structural features this approaches 
the larval form of Labrax lupus, as shown by Eaffaele * in his valuable paper, but the 
form of the fish appears to be more slender and elongated. 

Another unknown form (G), procured in March, is characterised by a similar position 
of the anus (about the middle of the body), and the presence of a large oil-globule towards 
the posterior part of the abdomen. No trace of yolk has been observed. t It is a slender 
and elongated larval fish, eel-like in outline, though the precise relationships are unknown. 
It occurs very regularly in March. The larval eel has not yet been described, and any 
suggestion on this head is conjectural. 

Cottus. — The youngest post-larval stage of a Cottus, which may be the earlier con- 
dition of Cottus scorpius, was procured at 4 fathoms in the mid-water net 4 miles off the 
West Rocks, St Andrews, 28th April 1887. It measured barely 6 mm.; the marginal 
fin is continuous, though there are indications of a differentiation anteriorly (first dorsal) 
and ventrally (anal). Two small flattened ridges indicate the ventrals. The pectorals 
are large fan-shaped organs. A few black pigment-specks occur over the brain, and a 
large dark band passes from the region of the pectoral to the anus on each side. The 
preopercular spines are developing. The notochord has a comparatively slight bend 
upward ; the hypural elements are in process of rapid development inferiorly, the region 
being distinct from the larval tail, which is still large, so that the organ is bifid. 

The next stage of this species was captured on the 22nd July 1887, in the mid-water 
net, at 33 fathoms, about five miles off St Abb's Head (W. by S.), in an area of 37 fathoms. 
The length was 7 5 mm. In this condition the young forms agreed with the older stage 
in presenting a pair of large tubercles on the nape, while the so-called anterior tubercles 
were represented only by a spine. Similar elevations occur in other Cotti, and are 
especially visible in young specimens. The black pigment-bands slanting downward 
and backward at the side of the abdomen are less marked than in the older form. The 
tip of the notochord is larger and more prominent than in the latter. 

In life this form had a nearly transparent body, with a series of black spots along the 
ventral margin from the anus to the tail. The cheeks and under surface of the snout 
were also dotted with black specks. 

Cottus quadricornis. — Examples (PI. XVII. fig. 11) of a somewhat older post-larval 
stage than in the preceding form occurred in Crail "Har'st" on 20th July 1887, on 
ground 15 fathoms deep, with the mid- water net at 13 fathoms, and subsequently in St 

* No. 125a, p. 15, tab. iv. 

t This form is described and figured in the Seventh Annual Report of the Fishery Board for Scotland, 1889. 


Andrews Bay. They were at once recognised by the deep black abdominal patch of pig- 
ment. The head is greenish, while the body is comparatively pale. The eyes are bluish, 
with a remarkable black St Andrews cross radiating from the pupil, the long axis being 
placed horizontally. Its length is 8 mm. Besides the conspicuous streak of black pig- 
ment which slopes downward and backward to the ventral edge, but does not meet that 
of the opposite side, stellate pigment-corpuscles occur on the under surface of the abdomen, 
a touch at the anterior region of the branchiostegal rays on the same surface, and a row 
along the ventral edge of the body above the anal fin. One or two specks also are present 
on the cheeks, and a considerable number over the brain, the latter being bounded poste- 
riorly by a curved line which joins a median black band in front of the dorsal fin. 

The four tubercles on the head are prominent, the posterior pair being the larger. 
The turbinal spines are not visible ; but the four preopercular spines are well marked, 
the superior being especially distinct. 

The first dorsal fin is only slightly arched, the second is continuous posteriorly with 
the larval tail-fin, which now lies at the upper angle, for the hypurals form a straight 
edge posteriorly. The permanent rays give a somewhat conical shape to the tail ventrally. 
The anal is likewise joined to the caudal; the pectorals form two fan-shaped organs, the 
rays passing close to the surface of the body — that is, the basal region is short, and thus 
in striking contrast to the organ in the larval Gadoids. The rays are massive though 
soft, and, as in the adult, present considerable free portions at the tip. The ventral s 
are small, and arise somewhat behind the bases of the pectorals. 

The next stage in the development of this species is illustrated by a specimen 18 
mm. long, procured in St Andrews Bay in the beginning of June. Traces of the St 
Andrews cross still occur in the eyes, the outer ends of the cross being most distinct. 
The head is of a dusky olive hue, with dull yellow over the brain, and the yellowish colour 
extends downward and backward to the upper abdominal region and in front of the 
pectorals, black chromatophores being studded on both regions. A dark belt passes from 
the first dorsal to the abdominal black band, where it ceases, then a pale bar intervenes 
between it and another broad belt occupying about two-thirds of the second dorsal (the 
anterior and posterior moieties being more or less free). A few blackish chromatophores 
occur dorsally, however, in the anterior moiety. No pigment is present in the tail 
beyond the latter region. The ventral portion of the abdomen is silvery ; the pectorals have 
dark pigment at the base of the rays, but no further. The ventrals are small and pale. 
The two dorsal fins are distinct, and the first has a small touch of pale pigment anteriorly, 
and a larger posteriorly. A slight marginal fin connects the last with the tail. The head 
presents the two large dermal processes on the occiput, and a smaller over each eye. 
The three opercular spines are prominent. 

Agonus cataphf actus, L. (Post-Larval stage). — A peculiar form (PI. XVIII. fig. 11) 
was procured in the mid-water net at 4 fathoms on ground 6 fathoms deep, St Andrews 
Bay, April 4, 1887, and is now identified as the young of the above species. It is remark- 
able for the great depth of both dorsal and ventral regions of the marginal fin, the outline 


being thus somewhat spindle-shaped. The snout is comparatively blunt, and the large 
size of the eye gives a resemblance to the condition in the Gadidse, and to some extent 
also to that in the Pleuronectidse. The auditory capsule is large, a prominent elevation 
of the outline occurring in the region. The jaws present the proportions in the groups just 
mentioned. The action of the heart is readily seen through the large opercular aperture. 

The body is elongate, about 7 mm. in length, the tapering extremity of the notochord 
being bounded by a somewhat lanceolate embryonic fin, in which the rays (embryonic) 
are developing next the base. The abdomen presents a marked incurvation in front of 
the rectum, and the anus is prominent. The pectorals are large and fan-shaped. 

The pigment is mainly of two kinds, viz., grass-green and black. The head shows 
black pigment over the otocysts, black and green in front of the eye, and on the branchial 
and mandibular regions. From the posterior margin of the opercular aperture to the base 
of the pectoral the same pigments occur. The pectorals have regular rays of similar pig- 
ment — tinged with pale greenish. The abdomen is covered with black and green pigments. 
The dorsal and ventral edges of the body have a series of black pigment-corpuscles, the 
former extending from behind the pectoral to the last vertical bars of the dorsal marginal 
fin, as shown in the sketch; while the latter extend from the vent to the narrow part of 
the tail. When viewed from the ventral aspect, a broad bar of pigment stretches between 
the pectorals, and a considerable quantity is scattered over the abdomen. In a smaller 
specimen the opercular fold is rendered distinct by the black pigment in front of it, and 
the pale region behind it. The deep parts of both dorsal and ventral areas of the mar- 
ginal fin have peculiar vertical streaks of greenish, and rows of black pigment-corpuscles. 
These touches are generally slightly curved, and appear to form two groups in each fin, 
a feature especially seen in the dorsal. The tail is faintly tinged with green. 

The gall-bladder is deep green ; the oil-globule is colourless. 

In the mid- water net, on 9th April, a few miles from shore, the next stage, fully 6 mm. 
long, appeared. In spirit it presents a few black chromatophores on the cheeks, and the 
bases of the pectorals show the black pigment-rows present in the older stage (PI. XVIII. 
fig. 11). The sides of the body from the tip of the pectorals backward almost to the 
tail show a series of isolated black chromatophores arranged in a double row, toward the 
dorsal and ventral edge. A group of similar pigment-corpuscles characterises the median 
or wide part of the marginal fin dorsally and ventrally. On the sides and ventral surface 
of the abdomen the same black chromatophores are scattered, and they run along the 
ventral surface to the base of the tail. 

The marginal fin has expanded dorsally and ventrally in the median line, but only 
embryonic fin-rays occur in it. A slight narrowing dorsally and ventrally is evident in 
front of the tail, which also shows embryonic rays. The anus is very prominent, and in 
front of it another projection of the edge occurs — a feature characteristic of the form. 
The large pectorals likewise show only embryonic rays, and the lines of black pigment 
spring from the distal edge of the basal process. The notochord is quite straight. 

The next stage observed was an example five-eighths of an inch in length, procured in 


the mid-water net on the 28th April. The chief coloration is fine chrome-yellow, especially 
on the pectorals, spicules, and dorsal fins (PI. XVIII. fig. 10). The pectorals are even 
larger, and the yellow pigment follows the line of the rays, with black points here and 
there, the intermediate region being yellow and black. The spines generally are large, 
those along the sides forming hispid rows, especially when looked at from the dorsum, 
and extending from the pectorals to the tip of the tail. The other features, which are 
chiefly those of the adult, have been indicated in a description published in the Fishery 
Board's Eeport by one of us.* 

Callionymus lyra, L. — The ova of the skulpin, both ovarian and mature, have been 
described in another place,t but they have not, as yet, been hatched. J By the aid of the 
large mid-water net, a large series of young examples of this species, ranging from about 
3 mm. to 10 mm. in length, were procured in August 1886. 

In the earliest stage (a little over 3 mm.) the body is characterised by the great size 
of the head and abdomen, and the attenuation of the caudal region. The head 
is very deep, and the projecting mandible passes upward at a marked angle. The 
premaxillary region is not yet much developed, and the mouth has not the protrusible 
character distinctive of the adult. The body rapidly tapers behind the abdomen, and 
forms a slender, straight, almost whip-like continuation bordered by the membranous 
larval fin, with its embryonic fin-rays. The head and body are speckled with brownish- 
black pigment-corpuscles, which attain their greatest development on the ventral surface 
of the abdomen — a part usually pale in fishes. The pectorals are small, and no ventral 
fin seems to have been developed. The armature of the operculum, so characteristic of 
the more fully developed stage, is as yet absent. The stomach at this stage contained 
fragments of minute Crustaceans, apparently Copepods. 

In the next stage, though the total length little exceeds the foregoing, consider- 
able progress has been made. A larger amount of pigment exists on the side of the 
body, especially behind the abdomen, and it extends, though sparsely, on the hyoidean 
surface in front. The premaxillary region is now slightly protrusible. The eyes are 
large and somewhat quadrilobate. The abdomen still projects prominently below ; 
but posteriorly the body has much increased in thickness, and the slender tip of 
the notochord, instead of being free, now forms the upper region of the caudal fin, the 
long inferior cartilaginous fin-rays stretching beyond it. The embryonic fin-rays extend 
from the notochord both dorsally and ventrally, and also at the tip. The hypural region 
is thickened, and the epiural is marked by a small patch. Speckled pigment runs along 
the base of the ventral marginal fin. The pectorals are somewhat longer than in the 

* Ann. Report for 1888, p. 267. 

+ Ann. Nat. Hist., Dec. 1885. 

+ Raffaele thinks that the reticulated condition of these ova (see p. 15) was due to immaturity — that is, that such 
represented the follicular layer of the ovarian egg, since in the ripe ova of G. festivus they were not present. Mr J. T. 
Cunningham, however, has recently met with a pelagic egg, off Millport, in June, agreeing quite with our former 
description, which was taken from mature eggs (vide, op. cit., p. 124). 

[Since this paper was read Mr Cunningham again confirms the original observation of one of us in 1885. — Jour. 
Mar. Biol. Assoc, N.S. i. p. 37.] 


last stage, and a pale process or papilla indicates the presence of the ventral fins. The 
next stage to be noticed (about 5 mm. in length) shows a more regular fusiform outline, 
and from the increase of pigment along the ventral surface it is considerably darker than 
the dorsal. By the widening of the cheeks the eyes have become more oblique, so that 
they are largely seen from the dorsum, whereas in the earlier stage they were more in har- 
mony with the usual piscine type. The forward growth of the premaxillary region, and 
the increased arch of the mandible, greatly alter the facial aspect. The ventrals appear 
as a pair of short fins below and in front of the pectorals. From the marginal fin the 
anal is differentiated inferiorly, and is separated by an interval from the caudal. The 
fin-rays are much more distinct in this lower fin than in the dorsal. The development of 
the hypurals has pushed the tip of the notochord upward, but it is still surrounded by 
the embryonic marginal fin. The inferior caudal rays far exceed it in length, and they 
spring from a vertical hypural edge. The shape of the tail is conical, broad at the base, 
and narrow at the tip. The opercular margin does not yet show spines. 

When 1 or 2 mm. longer, the arrangement of the pigment is unaltered, the ventral 
surface and posterior region of the body being tinted somewhat deeply by stellate 
pigment-corpuscles, while the dorsum is less uniformly coloured. The body behind the 
abdomen is thicker, so that when seen laterally the fish is fusiform, though, viewed from 
the dorsum, the head and abdomen are still disproportionately broad. The pectorals and 
ventrals are larger, the former showing blackish pigment-specks on the fleshy basal region, 
and a few dark radii on the fan-shaped distal region. The under surface of the ventrals 
also exhibits dark streaks between the rays, and they are considerably shorter than the 
pectorals. In shape both pairs of fins approach those of the adult. The first dorsal is 
merely indicated by a few short processes. The pale second dorsal fin (the embryonic 
fin having disappeared) begins behind the middle of the back, and terminates a short dis- 
tance in front of the caudal fin. From the anus the anal fin extends to a point below, and 
somewhat beyond the dorsal above, as shown by an imaginary vertical line. Between 
the rays very evident dark pigment-streaks occur, a feature in consonance with the 
development of pigment on the ventral surface. The tip of the embryonic tail has now 
coalesced with the upper lobe of the permanent caudal, and the latter is somewhat less 
conical as well as broader at its termination. In some of the more advanced specimens 
at this stage the opercular armature is present as a straight spine, with a spur — coming off 
nearly at right angles — toward the tip. 

When the length of 9 mm. has been attained, the body is still stouter behind the 
abdomen, the ventral fins have gained greatly on the pectorals, so that though they arise 
considerably in front of them, their tips are nearly in a line with the extremities of the 
former. The fan-like pectorals have much stiffer rays, arising from the semicircular 
base. The opercular spines stand out on a long process, and their tips slightly incline 
towards each other, the margin between them being semicircular. The first dorsal is now 
distinct, and the anterior rays of the second dorsal are longer than the others. 

When 10 mm. long, the characters of the adult are more evident, the telescopic mouth 

VOL. XXXV. PART III. (NO. 19). 6 S 


at once attracting notice. The median rays of the pectorals have lengthened, yet the 
ventrals are almost as long, the tips passing beyond the anus. The fleshy pad at the 
base of the pectoral is pigmented, but otherwise both these fins and the ventrals are pale 
in colour. The first dorsal is still short, but the second dorsal and the anal fins are 
prominent. The caudal expansion is now truncated at the tip. The abdominal surface 
is still coloured with black pigment. When taken out of the water the dorsum is 
somewhat greenish, diversified here and there with black pigment just as on the occipital 
surface, at the first dorsal, on the posterior end of the abdomen, and in the form of two 
conspicuous wavy bands behind, i.e., in front of the tail (vide PI. XIX. fig. 11). The 
figure here referred to is from a sketch made some days after confinement in a glass tank 
in the laboratory, hence the coloration is modified. The abdominal region is pinkish, 
from the contained food. The eyes are lustrous and greenish. In the laboratory the 
young fish lay at the bottom, keeping the pectoral fins in active motion, while the 
ventrals were spread out like a pair of wings. 

All the young forms above described were captured some distance from the bottom 
(though they occasionally occur close to the St Andrews rocks in August), and therefore 
the development of the fins after the disappearance of the embryonic membrane is in 
relation to this pelagic life. The remarkable duskiness of the ventral surface, which is 
pure white in the adult, is probably also connected with their temporary sojourn in the 
region above the bottom. The protective spines on the opercula are, it is interesting to 
note, very early developed. The vertebral column in the largest examples shows thin 
transparent ossifications of the surface and of the arches, but the centra are more or less 
notochordal. A specimen, 20*5 mm. in length, obtained from the stomach of a cod, still 
more closely resembled the outline of the adult, though the pigment had been removed 
by the gastric juice. The opercular armature of this example was well developed, 
presenting three large spines posteriorly (one passing backward and two upward), while 
one large and several minor spines occurred in front. We have found that the adults 
spawn in August, it may be somewhat earlier or somewhat later, and it is plain that 
all these young specimens cannot belong to the same period — cannot be, that is to say, 
merely a month or two old. Such a supposition would be inconsistent with what has 
been observed in the young from other pelagic eggs. On the other hand, if these are the 
young from ova spawned the previous August or thereabout, then their growth is some- 
what slow, but probably some examples spawn much earlier than others. 

Liparis montagui, Donov. — Shortly after hatching, which is easily accomplished even 
under unfavourable circumstances, the larvae (PI. XIII. fig. 1) move both tail and pectoral 
fins actively. The cuticle presents a finely reticulate appearance on the marginal fin, which 
everywhere has embryonic fin-rays, and a series of globular glands occur over this and the 
sides of the body. At a somewhat older stage (PL XVI. fig. 7) the yolk-sac is studded 
with stellate black pigment-corpuscles and touches of chrome-yellow, and on each pectoral 
is a large spot of yellow with black chromatophores, and then a narrow yellowish curved 
band with similar black pigment. A few chromatophores with remarkably elongate 


processes form in a line along the ventral edge of the myotomes, but none exist on the 
dorsal. The eyes have a greenish iridescence like a diamond-beetle's wing. The noto 
chord is quite straight at the tip, and its cells are large. The tail-fin presents only a 
trace of a dorsal enlargement. The nasal capsules form two conspicuous pits. The 
heart has a small oil-globule below it, and the large oil-globule lies immediately behind 
the heart — in front of the yolk, which is granular. No blood-vessel occurs in the tail 
proper, the aorta passing almost to the tip of the chorda, and bending upward and forward 
into the vein. The blood is comparatively pale, being only faintly pinkish in the 
heart. The vertical vessels (intervertebrals) running up the sides of the notochord are 
proportionally large. The branchial bars are cartilaginous. The liver is pale, and bur- 
rowed with large vascular channels, while the densely folded alimentary canal lies above 
it. It curves into the posterior region of the yolk-sac, and when the latter is viewed 
from the ventral surface a folded region of the gut is evident, but no external opening. 

The larval form figured in PI. XV. fig. 2, was supposed to be Montagu's sucker 
at an early stage, but it differs in the presence of dorsal pigment. Some variation, 
however, may occasionally occur. On the other hand, several species may have ova and 
larvae very similar to Montagu's sucker. In this form the regularity of the vitelline 
vessels and their simplicity, as well as the large size of the cephalic vessels, are noteworthy. 

Centronotus gunnellus, L. — Masses of ova* about the size of a Brazil-nut have more 
than once been found in cavities (holes of Pholas) at the Pier Pocks, with the parent- 
fishes coiled beside them. The examples specially dealt with occurred on March 14, 1887. 

The egg-capsules examined were somewhat friable, as all the embryos had escaped. 
The zona has a finely punctate appearance, and the punctures are most regularly 
arranged. The lacerated margin, however, presents very fine crossed fibres, probably 
due to the condition of the specimens, and these fibre-like markings disappear in 
Farrant's solution. 

The larva measures just over ^ of an inch (ihy)« I* is extremely translucent, 
and when hatched shows no trace of pigment, save that the pupil is dark (PI. XIII. 
fig. 6). It is extremely hardy and active, darting through the water in various 
directions, and again resting on the bottom. The head is blunt and rounded. The 
auditory organs (au) are very large, the eyes moderately so. The most remarkable 
features are the extreme length and thinness of the embryo, its eel-like form, the 
great length of the alimentary canal (g), and the character of the yolk-protuberance 
(y), which is directed somewhat forward. The latter is not of great size, and the 
vitelline mass proper is of an elongated ellipsoidal shape with a faint opacity, and 
having in its anterior ventral part a single oil-globule (og) of crystalline translucency, 
very slightly tinged with ochre, and surrounded by a thick protoplasmic coat (p); while, 
from the shortness of the sac, the globule is near the heart. Entirely covering the 
posterior surface of the yolk is the liver (Ir), which projects as a long cellular process 
from the abdominal region proper, and insinuates itself between the hypoblastic covering 
* These ova appear to have been first recognised by Mr Anderson Smith on the West Coast. 


of the yolk (y) and the thin yolk-sac (ys). The oil-globule (og) seems to lie in a pocket 
in the cortex of the yolk, as an equatorial line crosses over it, yet it is also enveloped 
by a complete protoplasmic covering (p). The gall-bladder (gb) occupies, in the upper 
posterior portion of the liver, a position just at the angle where the ventral embryonic fin 
(ef) joins the yolk-sac. As first shown by De Filippi,* the gall-bladder of the larval 
Clupea Jlnta lies also behind the yolk ; the liver, however, does not in this instance pass 
downward. The pectoral fin (pf) is somewhat fan-shaped, very thin and membranous, 
and stands erect ; while a chitinous clavicle (el) is fixed by a longitudinal lateral attach- 
ment. The long intestine, with a distinct rectal portion, curves downward a considerable 
distance from the yolk, and cuts off, by the anal protrusion (a), a ventral fin-area about 
one-third the length of the inferior embryonic membrane. The lumen of the alimentary 
canal is spacious and the walls much folded, but that of the oesophageal section is very 
much diminished. A valve or cincture marks the commencement of the rectum (hg). 
The segmental duct (sg) is plainly seen passing from a convoluted pronephric portion 
(with oval glomerulus, gl), with an undulating course to a spacious urinary vesicle (uv), 
which opens close to the anus (a). The notochord (nc) is fairly straight, but in the 
mid trunk it ascends by a gentle curve and gently bends round, to end between the 
posterior limits of the two eyes. It has the usual large irregular cellular structure, and 
presents a distinct perichordal sheath (pes). The heart (h, PL XIII. fig. 5) is fully 
formed, showing a rounded ventricle, which gives off, in front, a narrow bulbus directed 
upward, and behind receives the large vase-shaped auricle, opening into a wide sinus 
venosus. The pericardial chamber (pd) is large, and its floor is entirely free from the yolk. 

The oral and branchial cartilages are well developed (PL XIII. fig. 5). The mouth is 
widely open on emergence, and water freely enters. Only tremors of the mandible, 
however, are noticed. The mandible (mn) is as usual a massive cartilage with an 
enlarged articulating extremity, joined by two cartilages from above, the anterior or 
quadratopterygoid (ptg), and a more massive hyomandibular (hm), which springs by a 
large base from the floor of the ear-capsule (au). Four acutely curved branchial cartilages 
(bra) are present, and the long hyoid cartilage (hyd) ends in a copula, which projects as 
a nodular eminence on the under surface of the mouth. The maxillary elements cannot 
be made out, but the front margin forms an overhanging upper lip. The parts of the 
brain are well marked, — the fore brain (fb), on the lower anterior face of which are 
laterally placed the nasal sacs (ol), the pineal gland (pn), and the large dome-shaped mid 
brain (mb), with the cerebellar fold (cb) behind. The eyes have a silvery lustre, with a 
black pupil. A few black pigment-corpuscles occur over the anterior superior curve of 
the eye, and this region shows a fine green shade like malachite. 

The structure of the ear is very complex, and the two otoliths seem to lie in the same 
anterior ampulla. 

The marginal fin (ef) commences very gradually between the ears, i.e., posterior to 
he cerebellum, and it does not become very wide, although its depth is somewhat increased 

* Ann. des Sci. nat., 3 me s6r., vii. p. 66, pi. i. fig. 1. 


over the anal region and at the tapering caudal extremity. In this latter portion 
embryonic striations or rays (er) appear, and are especially distinct in the upper portion. 

In a fortnight the changes were the appearance of very finely stellate pigment over 
the corrugated rectum (hg), and between the latter and the urinary vesicle (uv). 
Smaller stellate pigment-spots proceed along the ventral surface of the gut to the yolk- 
sac, but they go no further. The same stretch behind the vent along the edge of the 
muscle-plates. Two or three stellate pigment-corpuscles also appear over the yolk-sac, 
about midway between the oil-globule and the notochord. In some the yolk has con- 
siderably diminished, and the oil-globule is thrown rather more to the front. The ridge 
from the ventral marginal fin goes a considerable distance forward over the yolk-sac. 

In a few days the young fishes exhibited a tendency to lie on their sides at the 
bottom of the vessel. 

After the lapse of five weeks, the majority of the larvae (PI. XIII. fig. 7) had the tend- 
ency just mentioned. A well-marked interrupted line of pigment runs from the cardiac 
region to the anus, passes forward and upward behind it, and is then continued to the tail. 
The marginal fin is continuous from the anus to the tail; a narrower fin occurs in front 
of this, and it diminishes about the region of the gall-bladder, which is large and distinct. 
The dorsal fin again is similar, and deepens only a little in front of the caudal, which in 
outline is somewhat lobate. The fin-rays are present in the tail, and are at this time 
better marked in the ventral (anal) than the dorsal fin. They are also distinct in the 
pectorals. The snout now extends forward about half the diameter of the eye in front of 
it; and the mandible projects a little further, but is motionless, the animal aerating its 
gills in its progress through the water. The lateral view of the head much resembles 
that of the cod or gurnard at a similar stage, but the diagnostic features besides those 
mentioned are the great length of the body and the median position of the anus. In the 
tail a hypural thickening has taken place, with a few coils of vessels which show pale 
blood coursing through them. The large size of the otocysts, and their continuation 
upward so as nearly to meet in the median dorsal line, is interesting. 

Most perished at the end of April from impurity of the water, but such as survived 
showed little change in habit and structure. 

Young gunnels, 1^ inch long, having the external features of the adult, were pro- 
cured in July. 

Lophius piscatorius, L. # — An injured example of the post-larval frog-fish was 
procured by means of the mid- water net, 15 miles off the Isle of May, on 30th August 
1886, at a depth of 25 fathoms, on 32 fathoms' ground. Its length is 7 mm. In outline 
it presents (PI. XIX. fig. 6) a large flattened head and a slender body, the notochord at 
the tip of the tail being bent upward at the dorsal angle. This curved terminal portion 

* The rarity of the floating ova of this species on the east coast of Scotland is remarkable, for the adult is very- 
common in stake-nets and trawls. So far as known, the spawn is also uncommon on the west coast, though there and 
off the south coast it has once or twice occurred recently. A specimen sent to the Fishery Board was stated to be the 
ova of the cat-fish. Raffaele failed to meet with it in the Bay of Naples. 


has still the embryonic fin, and the adjacent part of the dorsal fin is long. The fin-rays 
proper (inferior) are well marked and long ; and the outline of the tail is thus peculiar. 
The specimen, though apparently younger than the last stage figured by Alex. Agassiz 
(op. cit., pi. xviii. fig. 2), if the condition of the tail be a reliable point, yet diverges 
very much from it : still more does it diverge from the older stages of Gunther and Day. 
The head forms an expanded flattened plate, rounded in front from the marked 
premaxillary curve, and with a median notch; while behind lie the two nostrils, a little 
distance on each side of the middle line. The large eyes are situated more than their 
diameter behind the anterior margin of the snout, and less than four (about three and a 
half) of them would give the greatest transverse diameter of the snout. They are thus 
large and prominent, and look directly upward. The cranial and other cartilages have a 
covering of transparent hyaline bony tissue. Behind the eyes the body narrows to a 
slender region, which is barely twice the length of the head. This region still carries 
the embryonic fin dorsally and ventrally as a nearly uniform fringe, with the true 
fin-rays developing in it, though in the anal portion (ventral border) the marginal fin 
here and there retains its original condition. The other fins present in the specimen are 
a pair of enormous pectorals attached to the great shoulder-girdle, a plate passing 
transversely behind the branchiae. The ventrals are situated in the middle line just at 
the anterior end of the pectorals, but both the former are injured. They are very short, 
but the rays seem to have been broken; yet, allowing for this, they appear to be little 
developed. The anterior fold of the pectorals runs in front of these fins externally. 
The great changes that must ensue during development in regard to the gill-slits and the 
situation of the pectorals are noteworthy, while the ventrals remain almost in the same 
position. The under surface of the head is marked by the expanded hyoidean apparatus 
and the slender mandibular bars, the symphysis of the latter forming a sharp angle in 
the preparation. The gills are three in number, and the gill-slit is large and long, 
extending from the dorsal margin to the anterior ventral attachment of the branchiae. 
The branchiae have simple papillose processes. In the stomach were small Copepoda and 
larval Crustaceans. 

XII. General Eemarks ojst Post-Larval Fishes. 

Variability in the size of young marine fishes of the same season is one of the most 
conspicuous features in their history, and probably depends on the earlier or later period 
at which spawning takes place. Moreover, it is evident that in those species spawning 
by degrees considerable differences in size will occur, in the same fish, between the 
products of those ova shed first and those which issue last. Some species, for example 
the cod and the sole, seem to have annually a very extended spawning season, the sole 
commencing in May off the east coast, and continuing till August. The fact just 
mentioned is demonstrated in a single sweep of the mid-water net on suitable ground in 


autumn when swarms of little gurnards are captured, the smallest of which are very little 
larger than those reared in the laboratory, while others are three or four times longer. 

Moreover, experience of fishes such as the salmon, in which all the ova issue nearly 
simultaneously, shows us that the growth of the young of the same fish is variable, many 
being larger than others at a given time ; some, for instance, becoming smolts at the end 
of the first year, others not till the end of the second. Further, during the second year 
great disparity occasionally exists amongst the young fishes. 

When we come to survey the condition in the cod, the problem is more complex, since 
the material is less abundant and more difficult to obtain. Statements had been made 
by various authors about the life-history of the cod, but these were both vague and 
incomplete; Prof. G. 0. Sars, indeed, as already indicated (p. 153), was the first to 
produce a definite and satisfactory account of certain stages. He, however, found, no 
intermediate links between the larval form of 6 mm. and the post-larval form 24 mm. in 
length, the former occurring on 28th May, the latter on 12th June, and the most recent 
remarks of Ryder leave the same gap. The spawning period of the cod, therefore, would 
appear to be later in the Norwegian waters. It occurs as a rule in April in the British 
seas, though a margin must be given on both sides of this period,* and the larval cod 
abound in the surface-waters in areas frequented by the adults at the time. Towards the 
end of the same month, however, small Gadoids occur in St Andrews Bay, the least being 
about 6 mm., so that it is possible such represent a post-larval stage from early ova. 
Others again are double the length and upward, indeed there is great variety. These 
small forms are met with amongst the others throughout the summer, and generally occur 
in the mid- water net rather than in the trawl. On the 1st of June, however, the dis- 
tinctive coloration of the young cod (now ^-| in. in length) is recognised, in a rudimentary 
condition, and subsequently there is no difficulty in following it. They occur in the 
trawl, and at the margin of the tidal rocks.t Now, can we assert that all these are the 
young produced from the ova of cod which spawn in April? At first one of us was 
disposed to think that those which appear in shoals off the rocks in June, and which are 
about an inch in length, were those of the previous season, since it would be difficult to 
explain this remarkable rapidity of growth if the spawning period (viz., April) be correctly 
stated. Though the latter rests on proved observation, yet it must be borne in mind 
that the limitation of the spawning period to April is arbitrary, and it may only reach its 
culminating point then. This consideration, and the remarks formerly made as to the 
causes of variability in size, may, when coupled with great rapidity in growth, form the 
whole into one continuous series of young cod of the season. Such has been rendered 
more probable by the occurrence of the smaller forms early in April as well as subse- 
quently. Moreover, a change of area apparently takes place to some extent, since the 
mid-water net shows that these post-larval cod only appear in the Bay in April and May. 

* Vide "The Pelagic Fauna of St Andrews Bay," Seventh Annual Report of the Fishery Board for Scotland. 
f As shown in the Report on the Pelagic Fauna, the sizes of the pelagic young-food fishes captured in the mid-water 
net does not increase as the season advances, apparently for the reason that the large forms go downwards as they grow. 


The net is small enough to capture larval cod, but they are not a conspicuous feature, 
very few having been seen, and the intermediate stages between the larval form and the 
post-larval are difficult to obtain. The latter apparently tend shorewards in the end of 
April, not at the surface, but in the deeper parts of the water, many, indeed, being by 
and by caught on the bottom by a fine trawl-net. They sport about amongst the tangle- 
forests and shallow water and neighbourhood, and as they get older seek the deeper 
parts near the rocks.* They then, as Saks says, form shoals in deep water on the various 
fishing banks, large numbers being caught the following summer both by liners and 

That a migration occurs in other Gadidse is apparent when we consider the case of 
the ling. As a rule, the old ling frequents the deeper parts, yet the young, ranging from 
5 inches upward, are plentiful off the Pier Rocks at St Andrews, especially in their barred 
or tesselated condition. The post-larval haddock also would appear to frequent the 
deeper water, as also does the post-larval whiting, the latter occurring in considerable 
numbers south-east of the Island of May, and at a later period than the post-larval cod. 
When older, viz., from 3 to 6 inches, they are not uncommon in St Andrews Bay at low 
water. The larval frog-fishes and other types follow the same habits. 

On the other hand, some of the ordinary Pleuronectidse, e.g., the flounder, take a 
somewhat different course. The larval forms are pelagic on the sites t frequented by the 
adults, and then they gradually seek the bottom as well as the tidal margin, especially 
the mouths of streams, in May, June, July, and August. They may be found in lessened 
numbers there till well grown, so that the migration of this form is slight. The young 
plaice appears to follow similar habits, but the large adults seek the deeper water off 
shore. The same may be said of the turbot, the brill, and the long rough dab. The 
larval forms of the craig-fluke (Pleuronectes cynoglossus) frequent the ground occupied 
by the adults, and various stages may be secured by the same haul of the net. On the 
whole, then, the evidences of migration relate only to the passage in certain species of the 
post-larval stage to the shallower water, and the tendency of all the healthy larger forms 
to seek the deeper water. The latter feature, for instance, is observed in the halibut. 
It is apparent, however, that certain flat fishes, e.g., the "witch" or craig-fluke, so far as 
at present ascertained, are confined for the most part to the deeper water and soft ground, 
both in their adult and their younger stages ; and Miiller's topknot, the lemon dab (to some 
extent), and others probably agree with them in this respect. It appears to be a feature 
of moment that the post-larval forms of such as the " witch " and long rough dab swim 
somewhat longer on edge — that is, are larger fishes with great depth of body before the 
eye travels round, and before settling on the bottom. The post-larval stages of the 
flounders and plaice appear generally in April, and are about half an inch or less in length, 
their eyes as a rule being lateral in position; and as the season advances the left eye 
moves forward a little, and approaches the dorsal edge. They vary considerably in size 
at a given time, but not so much as the " witches " formerly alluded to, and would 

* Vide No. 104, p. 309. + Generally inshore. 


appear to seek the tidal margin with great rapidity. The flounder and plaice are 
probably the first to appear, the common dab being a little later. All the shallow sandy 
flats round the British shores abound with young Pleuronectids. 

The Clupeoids of the first series appear in great numbers in the Bay in March, and 
their presence corresponds with the period at which the hatching of the eggs of the herring 
was accomplished in the laboratory. The comparative rate of growth was followed there- 
after in the Bay throughout the summer months. Thus they measure about 7 mm. in 
the early part of March, 12 to 15 mm. a month later, and in two months about 20 mm., 
with a great increase in depth. In four months (i.e., from March) they reach 27 to 28 
mm., though such may be small examples of this (the first) series. Those of the second 
series occur in September and the following months. 

Though the differences existing between the larval herring and the larval sprat are 
marked at the period of hatching, the former being a much larger and more active fish, 
which soon gains strength to mount upward in the water, the latter being shorter and 
furnished with a larger yolk, yet the rapidity in growth soon obliterates the most evident 
features. Thus no bold line of distinction, as regards increase and safety, can be drawn 
between these closely allied forms with their dissimilar eggs. Each is as prolific as the 
other, and holds its own in every part of our seas. Much, no doubt, remains to be 
discovered in this and similar cases of divergence of the ova of closely allied species, but 
at present no general law can be laid down on this head, or in regard to the occurrence 
of oil-globules. It is difficult to explain why the brill and Miiller's topknot should have 
an oil-globule in their pelagic ova, and the turbot and " witch" be devoid of it. # 

The larval forms associated with the sand-eel occurred at the end of March, their 
length being 6 to 7 mm. At the same time, however, larger forms were captured, viz., 
18 to 25 mm. On the 14th April they had reached 8 to 9 mm., and at the end of the 
month 14 mm. at most. On the 7th May others measured 27 mm., 10th July 33 mm., 
and in August 45 mm. 

When placed under favourable conditions, there is no doubt young fishes grow 
rapidly, as in the case of the viviparous blennies, which before leaving the ovary of the 
adult reach the length of nearly two inches. 

While there is no difficulty in rearing large numbers of food-fishes to a certain stage 
in the laboratory, it is probable that it would be most convenient, when stocking 
certain bays, to place the larval fishes in the sea within a week, for thus they would be 
furnished with more abundant, more varied, and more suitable food. Further, the intro- 
duction of adults ready to spawn (e.g., soles) in suitable sandy bays, would probably be 
found more economical than the method indicated, and they can be carried long distances 
with ease and safety. The same remark applies to the herring, for adolescent examples 
accustomed by degrees to fresh water, can thus be carried without injury to distant regions. 

* Mr Cunningham (op. tit., 1889, p. 48) has recently broached the hypothesis that the presence of an oil-globule 
in the egg is connected with abundance of oil in the adult. This would not seem to suit in contrasting the turbot with 
the brill, the cod with the ling, the bib with the gurnard, the dragonet with the rockling, &c. 

VOL. XXXV. PART III. (NO. 19 ). 6 T 


Monstrosities. — Abnormalities of a marked kind were not frequent in the marine 
larval fishes hatched in the laboratory. Amongst these observed, however, were a 
double-headed example of Pleuronectes limanda (PL V. figs. 3 and 3a, in ovo), the 
latter figure being taken three days later than the former, and when the pigment had 
appeared. A cyclopean embryo of Trigla gurnardus, again, is represented in the same 
Plate, fig. 5, other abnormalities being present in the ovum. An abnormal tail of the 
same species, referred to elsewhere in this paper, is also shown on PI. XIV. fig. 3. The 
notochord with its investment continues in the axis of the body, while the caudal region 
bends separately to the left. 

XIII. Anarrhichas lupus, L. The "Wolf-Fish. Development and Life-History, 
with Remarks on the Salmon and other Forms. 

Intra-Ovarian Ova. — The intra-ovarian ova of the wolf-fish had been met with occa- 
sionally during the work for H.M. Trawling Commission, it being stated in the Report 
that they were of considerable size in February, and that at this period an abnormal ex- 
ample of a large ovum (which had not been discharged) was observed. In August, again, 
the ova were more than an eighth of an inch in diameter — in fact, it was clear that the 
spawning period of this fish was late, and in contradistinction to the published views on 
the subject. Thus, Day* mentions that, " according to Pennant, it spawns in May and 
June, when it deposits its ova on the leaves of marine plants ; the fry are of a greenish 
colour." Parnell, also quoted by this author, states that " about June the young are 2 
feet in length." t That it could not spawn in May or June was evident by an examina- 
tion of a specimen 3 feet 1 inch long. The ovaries were 6 inches long and about 1\ inch 
in transverse diameter as they lay on a flat surface, connate from the posterior end forward 
nearly a third of their length, and fixed by strong membrane in front of the fork. The 
ova form dense masses along the inner edge of each ovary, the great bulk of the eggs 
occurring there, for those on the rest of the surface were less numerous. The majority of 
the eggs are nearly of equal size, viz., about 1*5 mm., and each is invested by a vascular 
follicular membrane with very fine epithelium. Amongst these, however, many minute 
ova (visible to the naked eye) occur, and ranging from '75 mm. downward. In these the 
capsule is thick, the contents coarsely granular, and the nucleus large. A single large 
egg, about 6 mm. in diameter, and with an oil-globule 175 mm., was present, having 
evidently been retained after the others had been shed, as occasionally happens to other 
fishes — both marine and fresh water. The arrangement of the ova along the inner (i.e., 
median) border of each ovary was lamellar, masses of eggs hanging from the wall, so that 
it was on the whole roughly fimbriated. Minute blood-vessels covered each egg and 
the intervening membrane. In the very early stage the ovum lies in a capsule in the 
epithelial membrane of the ovary, and shows a large nucleus and nucleolus ; while the 

* Op. cit., p. 196. t Fishes of the Firth of Forth, p. 240. 


contents of the egg are finely granular. The usual changes take place, the nucleolus 
disappearing, and by and by, when the egg reaches 1 mm. in diameter, only coarsely 
granular contents are present. Before deposition the yolk clears up and the oil-globule 
becomes conspicuous. 

The earlier stages of the extra-ovarian development of the wolf -fish have not yet come 
under notice, for the ova procured on 16th January 1886 had reached an advanced stage, 
the embryos being considerably developed, and showing not only abundant pigment in 
the anterior dorsal region, and in the eyes, which had a silvery sheen, but an active vitel- 
line circulation. The movements of the embryo within the egg-capsule, too, were frequent 
and vigorous. It was necessary to tear the large mass of adherent ova in order to place 
them in the glass vessels of the laboratory, and a few embryos were thus set free. 
During the next four or five days many spontaneously emerged, but the appearance of 
those which escaped on the 23rd or 24th of January presented no noteworthy advance 
on their predecessors. The larval fishes at this date measured 1 1 or 12 mm. on emerging. 
The translucent body (PI. XX. fig. 2) is comparatively slender, and is surrounded by a 
delicate and continuous marginal fin. In the lanceolate caudal region a slight dilatation 
occurs. The yolk is bulky and of a translucent straw-yellow hue, and a large oil- 
globule of a dull yellowish tint is present. The coverings of the sac show finely granular 
cells with large nuclei (PI. XXI. fig. 5). The pectoral fins are in constant motion, just 
as those of the young salmon are, and yet it is doubtful whether the young wolf -fishes 
here referred to did not escape prematurely. Some had difficulty in escaping from the 
zona radiata, a circumstance noted, however, at a much later stage, when the head and 
yolk-sac frequently remained enclosed, while the tail alone was free. 

The favourite position of the most vigorous larvae is on edge (PI. XX. fig. 5), the 
rounded pendulous yolk resting on the bottom of the vessel, and thus steadying the 
young fish, while keeping its head above the sand or sandy-mud. The large oleaginous 
globule is situated on the anterior face of the yolk, a short distance below the head, and 
may possibly aid in maintaining the sac in the position just indicated. The situation of 
the globule is characteristic, for in no other British Teleostean has this precise position 
been noted, nor does Agassiz indicate it in any American form, though, in his conjoint 
work with Whitman, an unknown embryo (not unlike Cottus) is represented with a 
small oil-globule behind the cardiac region. F. Kaffaele,* however, has quite 
recently shown that certain Mediterranean forms, such as Mullus and Coris, have 
pelagic larvae which bear the oil-globule at the tip of the prow-like yolk. The 
feature is interesting, as this body in most forms in which it has been observed is 
situated towards the posterior border of the yolk-sac, as, for instance, in another form, 
viz., LophiusJ frequenting like Anarrhichas the sea-bottom. When viewed from 
above (PL XX. fig. 5), tbe globule projects just in front of the snout, the great vitelline 

* Abdruck aus den Mittheilungen aus d. Zool. Stat. Neapel., Bd. 8, Heft 1, pp. 20, 35, taV. 2, figs. 5, 6, and 
18, 1888. 

t It must be noted, however, that Lophius, unlike Anarrhichas, has a pelagic ovum (vide Agassiz " On the Young 
Stages of Oss. Fishes," Proc. Amer. Acad. Arts and Sci., vol. xvii., 1882, pi. xvi. fig. 3). 


vein passing upon the right, and in some a distinct bulging of the yolk-sac occurs in 
front of the globule. Moreover, during the extremely cold weather of the period, 
the globule was often thrust outward at this part. The head of the embryo 
presents a remarkably truncate appearance, the snout, indeed, projecting less than the 
large eyes, when viewed from the dorsum (as in the figure just mentioned). A decided 
difference exists, therefore, in this respect, when compared with the young salmon, in 
which the long yolk-sac trends downward and backward, whereas in this form the 
yolk-sac is globular, and is directed downward and forward. 

This being, so far as known, the largest British marine Teleostean larva yet 
described, its comparison with that of the salmon, the largest fresh-water form, is 
naturally suggested. Many points of contrast are at once presented ; thus the difference 
in coloration is marked, for while both have pigment in the eyes, the yolk in Anarrhichas 
is straw-coloured and inconspicuous, whereas in the salmon it is rendered conspicuous by 
the deep reddish-orange colour of the oleaginous globules. The latter become grouped, 
again, in masses in the upper region of the yolk-sac; whereas in Anarrhichas the 
single, large, somewhat lenticular globule maintains a constant position in front. The 
general pinkish tint of the newly-hatched salmon, further, is more pronounced. The 
external features of the wolf -fish also present less differentiation, for the marginal fin is 
simple and continuous, the tail is lanceolate, and the pectoral fins at this stage are much 
smaller. The head, moreover, is very different, for when viewed from the dorsum 
(PI. XX. fig. 5), the large eyes, as already noted, project even further than the snout ; 
while in the salmon the snout protrudes considerably beyond the eyes. 

In a newly-hatched example about a week later (1st February) the tail is somewhat 
bluntly lanceolate, the ventral lobe being more distinct than the dorsal (PI. XX. fig. 1), 
and the notochord passes in a straight line backward to terminate in a point. A slight 
wrinkle of the margin of the delicate caudal membrane occasionally marks this termina- 
tion. The notochord here is finely cellular, the size of the cells increasing anteriorly. 
The neurochord presents a marked diminution just above the margin of the vascular loop 
beneath the notochord. The membranous tail-fin has fine striae (embryonic rays, 
produced by delicate fibres), and is cellulo-granular. The aorta, in coursing back- 
ward, gives off a twig inferiorly, which diverges from the ventral surface of the noto- 
chord, and then ends in a slender vessel passing almost to the tip of the chorda, and 
returns as a recurrent vein. The ventral twig of the aorta splits into two loops, which 
ramify over a limited area (see PI. XX. fig. 1), and then joins the caudal vein by a single 
trunk. This condition diverges from that in the salmon at the same stage, but a com- 
parison between the two forms may be conveniently reserved till a later period. The 
circulation in the caudal region attains but a slight development in the wolf-fish at this 
stage, whereas the vitelline vessels are as fully developed as in more advanced 

Vitelline Circulation at the Period of Hatching. — We have already mentioned that 
on emerging a large vitelline vein passes on the right side of the larval trunk, collecting 


the blood returning by two main branches — the afferent trunks from the liver carrying a 
stream downward posteriorly and joining the posterior division, the other great branch 
receiving its supply from the opposite side (PL XX. figs. 4 and 5). Considerable gaps, 
as regards the larger trunks, occur on each side of the vitelline vein and below its branches 
at this stage, but examples somewhat differ from each other in this respect. These 
trunks are all, even the smallest, of comparatively large calibre, and appear in ordinary 
views to be hollowed out of the yolk, and without the distinct walls of typical venous 
trunks. The vessels frequently anastomose with each other, and their general direction 
is downward and forward. Some of the upper twigs (in larvae of 22nd February) pass 
rapidly across the yolk-sac to join the main vein, above the oil-globule. When the 
fish is in a dying condition the current becomes less swift, and frequently recoils in the 
vessels, but especially in the great vein. On the left side (PL XX. fig. 2) a great trunk 
from the liver courses along the anterior hepatic curvature, while several smaller but 
still considerable trunks issue from the same organ posteriorly, and rapidly break up into 
many branches, forming a complex network over the posterior half of the sac. Their 
terminal branches join a large and nearly horizontal trunk, which slants slightly upward 
and forward to join the vein. The divergence of this arrangement from such a species 
as represented in PL XV. fig. 2, from ova resembling those of Montagu's sucker, is 
marked, the vessels in the latter being few and short, and having a comparatively 
straight course. 

About a month later (PL XXI. fig. 2) the complexity of the circulation on the left 
side has increased. The anterior vessel of the series issuing from the liver, which curves 
downward and forward — branching as it goes, has become much larger. 

On 1st April, again (about 5 weeks later), the chief change since the former period is 
the great size of the anterior trunk (marked a, PL XXI. fig. 2) and its shorter course. 
The vessels to the right (that is the posterior vessels) are diminished, and they have a 
decided slope to the front. The slope, however, is much less marked than on the right 
side. In the most advanced forms at this time (PL XXIV. fig. 7) the alteration in the 
direction and size of the vessels of the yolk is remarkable. Thus, on the right side, the 
main part of the blood is conveyed by a large trunk passing to the posterior border of 
the liver — slightly downward and forward (though the course may vary) into the great 
vitelline vein, a little below the heart. This blood does not, therefore, pass over the sac. 
Some of the vessels of the yolk seem to have diminished in size. The great trunk just 
alluded to is twice the size of that in an example in which the yolk is still very large 
(i.e., a younger specimen), yet each, though so unequally advanced, may have emerged 
from the ovum almost at the same time. Of course, the diminution of the yolk-mass 
enables the larval-fish to swim more readily through the water. It must be noted that 
the direction of the trunks detailed above may, as indicated, vary considerably, for in one 
example two large trunks issued from the front of the liver, then rapidly curved toward 
the anterior border and joined the efferent vein. The rapidity of the current in these 
two main trunks is in contrast with the steady and slow current in the branches covering 


the surface of the yolk, and though a large posterior vessel shows a more rapid flow than 
the rest, still it is not so swift as in the anterior trunk. The great anterior vessels 
have a shorter and more nearly horizontal course, and do not wander over the yolk as 
observed in February. 

The blood-discs of the wolf-fish present no special feature of note, though they some- 
times undergo peculiar changes of form after escape (PL XXL fig. 5a). 

In about a fortnight after hatching, the yolk-sac has materially diminished, forming a 
rounded projection anteriorly, somewhat less in bulk than the head above it. In the ex- 
ample studied, the globule was found to have passed to the right side (PL XXI. fig. 3), 
and a considerable portion of the yolk-mass lay in front of it. The vitelline vein was thus 
carried backward by the peculiar displacement of the oil-globule. The smaller vessels 
curved round the anterior border of the globule to join the vein or its branches at various 
angles, but in the main more or less transversely. A similar arrangement occurs pos- 
teriorly, though the branches may, in lateral view, often have a shorter course, since the 
great vein is, inferiorly, nearer the posterior than the anterior border. On the left side 
(PL XXI. fig. 3a) the anterior vessel is now superior in position, and is somewhat trans- 
verse in direction, while the other branches are much shortened, and their course is 
chiefly downward and forward. In those most advanced (PL XXVII. fig. 1) the yolk- 
mass is noticeable only as a projection in front, for the posterior end merges, as it were, 
in the abdomen. The oil-globule now lies under the branchiostegal rays. It is note- 
worthy that the absorption of the yolk was accomplished in some cases at the beginning 
of May. Moreover, while the young fishes were comparatively delicate in their earlier 
stages, an alteration seemed to ensue about the time of the absorption of the yolk, so 
that they became more hardy — their tenacity of life being so great that examples 
appeared to surfer little though left unchanged for some days in a very small quantity 
of sea- water. 

We have explained that the outline of the yolk-sac in Anarrhichas is quite different 
from that in the salmon, being spheroidal instead of elongated and sloped posteriorly. 
It corresponds, in fact, rather to the condition in the embryonic salmon before hatching 
— say the 40th day in those which hatch on the 60th day. The shape in the salmon 
also shows the changes of outline during absorption more boldly, the sac in the healthy 
fish (PL XXII. fig. 4) becoming gradually attenuated posteriorly (Ibid., figs. 7 and 8), and 
occasionally in the more vigorous specimens, as Sir J. Gibson Maitland has shown, a 
portion of this region is constricted off and shed. Such a condition is not possible in the 
spheroidal yolk of Anarrhichas. Further, the great abundance of oleaginous globules in 
the upper part of the yolk in the salmon, and the occurrence of smaller globules through- 
out the entire mass, is wholly unlike Anarrhichas. In the latter species the single large 
oil-globule is nearly constant in position, but in the salmon this is certainly less so, though 
the oleaginous spheres are towards the upper region, and often on the right of the embryo, 
yet during absorption of the yolk they become more or less posterior in position. At no 
period does the globule in Anarrhichas pass backward other than the slight degree shown 


in PL XXL fig. 3, a feature probably due to position under examination. The relation of 
the large globule to the position of the respective larvae when at the bottom of the water 
is not perfectly clear, though there is reason to believe that, in Anarrhichas, as already 
pointed out, it is directly connected with the attitude assumed by the larva when at rest. 
Young salmon lie on one side amongst the gravel in their early stages, or occasionally 
rest with the yolk-mass dipping between the pebbles on the bottom, so that a definite 
position of the oil-globules in front would appear to be of little importance. The 
contained fluid, or deutoplasm, of the yolk-sac seems to be similar in both species. 
When discharged into the water it is transparent, viscous, and very tenacious. After a 
time it becomes solid, and of an opaque white hue like a stratum of tallow. In the sal- 
mon it presents in the latter condition abundance of dense oily globules, with adherent 
granules, resembling nucleated cells. In the large globules, however, the granules form 
only a superficial fringe. In some cases the oily matter, on escaping, sinks to the bottom of 
the water. Two kinds, indeed, of this matter are present — (1) orange-tinted oil, which 
floats at the surface, and (2) minute particles of oil imbedded in and held down by 
granular substance. When pressure is applied to the large globule it divides into two 
or more portions, so that it would appear that no definite protoplasmic investment 
encircles it. It also occasionally passes to the fundus (PL XXII. fig. 7), and sometimes 
its surface is slighted fissured. Some of the oil-globules appear paler in colour than 
others of the same size. The large globule is observed to persist almost till the 
posterior process disappears from the yolk-sac (PL XXII. fig. 9). Externally the latter in 
the salmon is covered by a layer of nucleated tesselated epithelium, the nuclei having 
nucleoli, and beneath is a fibrillated coat, below which the vascular layer lies. In hernia 
of the yolk-sac, the free portion presents a striated appearance, due possibly to the 
protrusion of the vascular (yolk-hypoblast) through the non- vascular layer, for in one 
example blood-vessels proceeded quite up to the margin of the hernia. 

The walls are contractile, for the sac shrinks towards the body of the embryo on the 
escape of the contents. So also the walls contract during the gradual absorption of the 
contents of the sac, as observed at the end of the first week (PL XXII. fig. 6). As the 
organ shrinks, obliteration of the smaller and then closure of the larger vessels takes 
places. By and by (about the 13th day) the anterior region of the sac becomes flattened, 
so that it forms merely a slight swelling on the surface ; and sometimes a few carunculi 
appear at the fundus. At the end of the third week the vitelline vein as well as the 
yolk-sac is much diminished. About the end of the fifth week the latter forms only an 
abdominal swelling, and is streaked with bars of pigment, which are directed downward 
and backward. 

A glance at the figures of the sac during its later stages in Anarrhichas will show 
that the absorption takes place in a different manner from that just detailed, since in- 
stead of the final prominence being posterior as in the salmon, it is in Anarrhichas 
anterior, and the large oil-globule in the latter likewise is in the same region. 

In regard to the circulation in the yolk-sac, the great vitelline vein in the salmon is 


in front, and in its course upward it receives several large branches, but in the wolf-fish 
it is generally on the right. Shaw's figure # shows the vitelline vein in the salmon 
occupying an unusual position ; the yolk-sac, moreover, is too small for a salmon one 
day old, and the oil-globule should either be larger, or be represented by several smaller 
ones. The general direction of the small branches is transverse or oblique, the upper- 
most appearing at the posterior margin, and coursing obliquely downward and forward, 
the middle being nearly transverse, and the lowermost transverse and then upward. 
The last main branch collects the blood from the upper part of the sac, and enters the 
great vessel from behind, not from the ventral side.t In Anarrhichas, on the other 
hand, the left side has its twigs mainly at right angles to the body. The two large 
trunks of the vitelline veins pass from behind forward and upward to form by their 
junction the great venous trunk, and it is their disposition that gives a character to the 
vitelline circulation, in contrast with that of the salmon. The afferent vessels stream 
downward into these on both sides of the sac, those on the left, however, entering the 
great branch by numerous trunks (PL XXI. fig. 2), and thus forming a contrast to the 
right side (PL XXII. fig. 3). It is interesting that, after forming a rete on both sides, 
the smaller trunks should again join to form larger vessels which empty themselves into 
the main branch of the side, as shown in the figures above mentioned. The current in 
the smaller wavy trunks becomes slow, thus probably enabling those changes between 
the contained blood and the neighbouring parts to go on efficiently, and on being accom- 
plished the rapid return of the blood to the heart is facilitated by the formation of the 
larger secondary trunks which join the great veins on each side. This arrangement is not 
seen in the salmon, though the vessels do not branch much, and enter the vitelline veins 
by a current not more than two or three blood-discs broad. In both species the supply 
to the sac is posterior, while the returning blood passes anteriorly. 

The absorption of the sac in the wolf-fish took place about the middle of May, so 
that in all probability it occurred at a period similar to that in the salmon. The 
difficulty in preserving special examples, and the great irregularity in the conditions at 
birth, made the exact determination of the period uncertain. The young wolf -fish, like 
the young salmon, exhibits increased swimming power as the sac becomes less, shooting 
upwards into the water at first with a wriggling motion, but later as the sac diminishes 
it swims more steadily. 

In the partial stasis preceding death the vessels of the sac are greatly enlarged, so 
that either the walls of these canals are contractile, or there is a great pressure of blood, 
and the latter certainly occurs from the slow rate of progress along the vessels. 

Circulation in the Trunk. — The circulation in the larvae which emerged in the 
middle and towards the end of January showed for the most part the ordinary Teleostean 
features. The main points will be detailed in comparison with the salmon, at a somewhat 
later stage, and, meanwhile, the condition on emerging may briefly be indicated. 

* PI. xxii. fig. 2, Trans. Roy. Soc. Edin., vol. xiv., 1840. 
t Vide Quart. Jour. Micr. tfci., vol. viii. N.S., pi. iii., 1868. 


The contraction of the auricle sends the blood into the ventricle, and the latter by 
the bulbus drives it into the four branchial arteries, which terminate in the dorsal aorta. 
The latter passes backward almost, though not quite, to the tip of the tail. Before reach- 
ing its termination (PL XX. fig. 1) a twig leaves the artery and goes into the lower lobe of 
the tail, forming, with the returning veins, an arrangement of at least four loops. More- 
over, the caudal vein bends downward, producing a loop or diverticulum just where the 
arterial twig leaves the aorta, and receives the branches returning from the region. The 
four loops referred to form a fan-shaped arrangement, one loop being in front, two median, 
and one posterior. In its course along the under surface of the notochord, the aorta sends 
a twig (Owen's intercostal) upward at each myotome, and it alternates and inosculates 
with the veins returning to the posterior cardinal. A specimen showing these features 
was found so late as 1st March, and had not long emerged, so that a margin for variation 
must be made. The artery on passing over the urinary vesicle transmits on the left 
side a large trunk to the rectum (a, PL XXVI. fig. 2), which by and by divides, a large 
branch sending twigs posteriorly, while the main vessel proceeds along the superior edge, 
giving off branches from its ventral margin. These pass downward and join the great 
portal vein (PL XXVI. figs. 1, 2, pv), which slants upward on the right side of the gut to 
proceed to the liver. The forward current in the arterial trunk (PL XXVI. fig. 2) goes 
a little distance beyond the point marked b to the point indicated by x, where it is met 
by an opposing current from the artery d. This last current shows a distinct pulsatory 
movement (as in vigorous arteries). Thus, at the point of contact, there is a neutral zone 
which occasionally is thrown a little backward so as to impinge by jerks on the forward 
current. Any difficulty arising from the presence of two diverse currents in the con- 
tinuous vessel is obviated by the ready exit along the comparatively large inferior twigs 
proceeding to the intestine. Streams of blood thus pour all over the intestine in a 
downward direction, and are collected by the great subintestinal (portal) vein. The vessels 
from the anterior artery (see Fig. 2) curve downward to the lower border of the gut, and 
as the intestinal vein in this region is above the lower margin 
of the latter, the branches going to it therefore curve upward. 
On reaching the liver the great intestinal (portal) vein breaks 
up into many branches, and from the margin of the organ large 
vessels pass to the posterior region of the yolk-sac (PL XXI. 
fig. 1, yvs), where their course has already been described. The 
cranial circulation is not readily made out. A large curved trunk, fig. 2.— Anterior curve of the portal 
the hyoidean (PL XXI. fig. 4, cv), leaves the anterior branchial 

vessel near the fork of the jaw, forms a loop behind, and passes forward between the eyes. 
Its course is hidden by the pigment of the latter. These vessels are proportionally larger 
than in the salmon, and the same may be said of the jugulars, which are in intimate re- 
lation to the otocysts. Such variations are probably associated with the differences in the 
configuration of the parts in the respective species. The ventral branch resulting from 
the union of the hyoidean and two first efferent branchial arteries, and which supplies the 

VOL. XXXV. PART III. (NO. 19). 6 U 


ventral fins of ordinary Teleosteans, is absent in the wolf-fish, which has no pelvic fins. 
The carotids pass along the base of the brain to the front of the snout, and the venous 
blood is returned by two large trunks, vn (jugulars), descending at the posterior border 

of each ear (cm), and joining the anterior cardinals to form 
the ductus Cuvieri (see Fig. 3). Avery distinct, though small 
vessel (ophthalmic), sends a swift stream of blood backward 
over the eye. 

If we examine the circulation at a stage two months later 
than the foregoing period, its complexity has considerably in- 
creased, not only in regard to the vascularity of the branchial 
lamellae, but also by the great development of the vessels 

Fig. 3. — Great vessels near the heart. . 

Right side. above and below the vertical aortic trunks. The vessels are 

more numerous than in the salmon one day old, and extend 
beyond the muscle-plates into the marginal fin dorsally and ventrally. 

The action of the heart is interesting, and in an example observed on 1st April it 
was as follows : — The sinus is distended with blood mainly from the large vitelline vein, 
then the auricle fills, and its contraction distends the ventricle. The contraction of the 
latter, again, expands the bulbus, dilating every crevice. Sometimes, as in the salmon, 
the ventricle does not quite empty itself, a feature due probably to the structure of its 
reticulated muscular walls. In the salmon, when the chamber is distended, and just 
before contracting, processes of the red fluid dip into the whitish walls, and show that 
even at this early stage the cavity contains muscular bands and interspaces. In weak or 
dying specimens of the salmon the auricle contracts more sharply than the ventricle, the 
latter having a slower vermicular motion. The current in the large venous trunk 
(cardinal), just before the contraction, often gives a jerk backward, this recoil being 
apparently due to the valves of the auricle, and its effects are visible in the 
remotest part of the venous system, especially in the sac near the base of the tail. The 
shortening of the auricle, a most marked movement in contraction, is towards the 
ventricle — just as the hand would squeeze an elastic bag chiefly at its fundus in order to 
drive the fluid by a jerk out of the muzzle. In these young fishes this organ is the 
ultimum moriens. 

In the salmon of the sixth week the aortic bulb is covered with pigment-corpuscles, 
apparently in the pericardial serous membrane, which elsewhere contains similar pigment. 
A band of muscular fibres is connected with the bulb a little way up, and the fibres are 
probably the same as those observed at the side of the pericardium anteriorly in Anar- 
rhichas. The contractions of the heart are most favourably observed from the left side. 

The heart of Anarrhichas in January had comparatively thin walls, which showed, 
in section, few fibres, but many nucleated cells. The thick region was towards the 
bulbus, into which two conspicuous valvular folds (aortic valves) project. In section, the 
area of the entire organ is cellular, with the exception, perhaps, of the external fibrous 
investment, the cells being apparently bound together by a protoplasmic matrix. This 


condition is precisely similar to that in the young Clupeoid, \ inch in length, the heart 
being saccular, and consisting of 3 or 4 layers of cells, many of them very round and 
prominent. The endothelium is, however, flattened. The cells in the wolf-fish project 
internally, so as to occupy a considerable portion of the central area, but much less now 
than afterwards. The bulbus in contraction at this stage is nearly cylindrical, its central 
cavity being evident just above the valves, and it is on minute examination seen to be 
composed of fibres and cells. The auricle has a cellular wall, the cells apparently being 
also fibre-cells, and the wall is now better defined externally, as if from more continuous 
fibrillation, this differentiation of a stronger outer layer being also observed in the young 
Clupeoid at the stage just referred to. 

As the organ attains greater complexity, the ventricle largely assumes a reticulated 
appearance from the vascular ramifications of its walls (PL XXV. fig. 1, ven), the whole 
being in many sections not unlike a fibrous sponge — a condition characteristic of the heart 
in the adult. This structure is also shown in the young Clupeoid, when fa inch long — 
small spaces being formed in the loose saccular walls, which, as in the slightly younger 
stage, are studded with large scattered nuclei. Quite different is the structure of the ven- 
tricular walls of the post-larval goby, g 5 ¥ inch long, the spaces being formed in very dense 
and thick tissue. The walls of the auricle in Anarrhichas, as in Clupea, Gobius, and 
others when \ inch long, consist of a thin muscular layer lined by endothelium. In the 
most advanced stage of the wolf -fish (20th June) the two valves at the base of the bulbus 
are well defined. The reticulations of the conical ventricle are much finer and more num- 
erous, and the fibres more distinct. With the exception of the central chamber, which 
occupies about a fourth of the thickness towards the base, the whole organ is reticulated in 
this manner, and the blood passes into the reticulations. So accurately do the auriculo- 
ventricular valves close that in the preparation a thick column of blood projects into the 
auricle, the mass being covered by a tense membranous layer, apparently valvular. 

The heart in the young salmon, at thirteen and forty-five days respectively, presents 
corresponding features to that of the wolf-fish, though the nuclei in the muscle-cells are 
much more distinct both in the early and later stages. 

When fully developed the circulation in the larval wolf-fish presents, in regard to the 
vertical or what may be called the vertebral branches of the aorta, a similar arrangement to 
that in the salmon, which has about 26 or 27 of them. These somewhat differ in the latter 
fish in the several parts of the body. Thus the anterior branches (PL XXVIII. fig. 2, a) 
course straight up from the aorta to the middle of the trunk, then give off the oblique 
twigs (6) which proceed downward and backward so as to form the oblique lines so notice- 
able in the living fish. They alternate with venous trunks (c and d) having the same 
direction. These arterial twigs admit only a single line of corpuscles, which proceed with 
great rapidity, and join the respective veins (c), and then by the larger trunk (d) debouch 
into the cardinal. Beyond the oblique arterial branches just noted the vessels course 
upward and slightly backward to the border of the muscle-plates, and give off various twigs 
before terminating in the venous radicles. Posteriorly the oblique branches appear to 


be less complex, and finally they form simple twigs towards the termination of the 
chorda. The arterial branches along the sides of the body appear (in optical section) to 
be more deeply situated than the veins. 

At this time no blood-vessel extends beyond the line of the muscle-plates — including 
those of the tail ; indeed, it is not till the third day that three or four loops of capillaries 
(PL XX VIII. fig. 3, ax) pass from the distal ends of the vertical vessels in the region of 
the adipose fin. They increase in number about the tenth day, yet at this period none 
occur in any other fin, except the tail. This is a noteworthy feature, though it is in 
consonance with what Alex. Agassiz has generally observed, viz., that the posterior 
dorsal is the first to be differentiated.* 

In the w r olf-fish the anterior region of the intestine (PL XXVI. fig. 2) is supplied by a 
large artery (mesenteric) d, which leaves the aorta on the right side, proceeds towards the 
intestine, and bifurcates after a short course. The superior branch runs along the posterior 
third of the upper part of the gut to inosculate with the main artery from the posterior end, 
as formerly mentioned. The other trunk passes to the lower side of the gut, and gives 
twigs anteriorly as previously seen, and these terminate in veins passing upward to the 
intestinal (portal) vein. A hepatic branch is also sent to the liver. Another branch (the 
subclavian) is given off by the aorta in front to each pectoral. In the salmon, again, besides 
this branch, the aorta sends twigs to the right and left sides of the yolk-sac. These, how- 
ever, were not noticed in Anarrhichas, though they may have been present. When about 
a week old, moreover, minute branches occurred in the salmon in the space between the 
yolk-sac and the ventral fin. Some of these seem to come from the arterial twigs of the 
sac, others from the oblique twigs on the side of the body. In the newest and most 
minute capillaries in the developing fish (salmon) the blood-discs pass edgewise, and at 
intervals, the rest being simply a flow of pale fluid: nothing else at least could be observed. 

Caudal Circulation. — The circulation in the tail of Anarrhichas, at the stage shown 
in PL XXVII. fig. 2, may very well be contrasted as regards the arrangement of the 
vessels with that in the salmon two or three days old (PL XXVIII. fig. l), though it 
has to be borne in mind that the presence of the fairly formed cartilaginous hypural 
elements in Anarrhichas has an influence on the relations of the parts. The aorta (ao) in 
the wolf -fish leaves the notochord just at its dorsal bend, a twig passing up, over the 
chorda, a little beyond the point of departure. The course of the vessel is backward and 
downward between the upper and lower hypurals. The next branch is a slender twig 
(cad) which passes, with a nearly equal interval on each side at its origin, almost to the 
tip of the notochord, where a few coils occur. It then enters the venous system. It is, 
probably, the representative of the small artery which, about a fortnight before, went 
upwards along the notochord and returned by a similar vein, as in the salmon. 

At the posterior termination of the hypural fissure the artery splits — a branch 
proceeding dorsally and another ventrally — the two lying nearly in a continuous line. 
From these a series of more or less parallel loops pass outward and obliquely downward 

* J'roc. Amer. Acad. Arts andSci, vols. xiii. and xvii. 


in a fan-like manner into the caudal expansion, the whole having a somewhat semi- 
circular outline, since the dorsal and ventral branches are less prominent than the 
median. A considerable margin of the tail is devoid of vessels. A downward loop is 
usually formed by the vessels just described, which enter the vein, and various inoscu- 
lations also occur terminally. The terminal, dorsal, and ventral series present 
slight irregularities, the dorsal loops especially being broader and less definite. The 
venous twigs from the arteries just alluded to pass forward in a similar direction to the 
two large venous trunks running along the posterior border of the hypurals. In regard 
to the axis of the body, these veins, as well as arteries, lie at an angle of about 45°, 
the ventral edge, moreover, having twigs which curve even further forward. By the 
union of the foregoing venous ramifications, the great caudal is formed. It lies beneath 
the aorta, and follows the same curved course as the artery. 

The circulation in the tail of the wolf-fish exhibits, at this advanced stage, a com- 
plexity similar to that in the salmon two or three days old, and the contrast between 
their rate of development is thus very great (conf. PI. XX. fig. 1, wolf-fish newly 
hatched, with PI. XXVIII. fig. 1, salmon newly hatched). It is unlikely that the 
development of the former in the egg is much more rapid than that of the latter, yet 
the wolf-fish on being hatched presents only a few loops of vessels below the straight 
notochord, while the salmon has attained a complexity of organisation in regard to the 
vessels and the curvature of the notochord only reached by the former two months later. 
Moreover, the nearly horizontal direction of the median vascular loops in the salmon is in 
contrast to the obliquely downward direction of those in the wolf -fish. It must, however, 
be remembered that the influence of altered circumstances (e.g., those in the Laboratory) 
upon the eggs of Anarrhichas is unknown : yet other eggs with thick capsules do not 
seem very readily affected by such external conditions. 

The size of the arterial trunks in the tail of the salmon seems to be smaller in com- 
parison with the veins than in the wolf -fish, and, moreover, the former has a large venous 
sac (comparable to the caudal heart of the eel) at the upward bend of the notochord, 
forming a large ovoid dilatation sloped upward and backward, and with a siphonal bend 
dorsally, and a slight contraction ventrally (PI. XXVIII. fig. 1, cvs). No pulsation is ever 
present in it, except the impetus from the pulsations of the arteries and from the action of 
the auricle. Stasis very readily affects this sac. In the somewhat older larva as much 
blood passes by the vessels beneath as through the sac in these circumstances. The sac seems 
to diminish rapidly, for it is indistinct on the thirteenth day, though the increased amount 
of pigment renders accurate observation difficult. When about a week old, a secondary 
enlargement in the form of a rounded sac sometimes occurs just behind the former. 

In the more advanced condition, as at the end of March, the vessels in the tail of Anar- 
rhichas (PI. XXII. fig. 2) have become much more elongated, and more definitely arranged. 
Further, instead of the obliquely downward direction of the main vessels, the whole form 
a fan-shaped arrangement, the median being horizontal, as in the salmon on the first day. 

The two main branches which respectively leave the aorta and join the caudal vein 


have now assumed a nearly vertical position, instead of being directed from above down- 
ward and forward. This position is due to the changes in the upward bend of the 
notochord, and the relations of the hypural elements. The regularity of the vascular loops 
towards the tip of the tail is noteworthy, each corresponding with a prominent cutaneous 
frill or process of the marginal (caudal) fin. 

Pigment. — On emerging from the egg, about the beginning of February, the larval 
Anarrhichas shows finely stellate black pigment on the head and other parts, especially 
along the dorsal region of the pronephros and alimentary canal, and behind the pectoral 
fins (PL XXI. fig. 1); a considerable number also occur on the adjoining region of the 
yolk-sac. It so rapidly increases that a conspicuous blackish band soon stretches from 
the pectoral fins to the anus. The pigment just described as passing along the dorsal 
line of the alimentary canal is found in section to be deeply seated, and to be scattered 
near the commencement of the segmental duct — on each side of the aorta and below the 
notochord. The pigment dips between the involutions of the duct anteriorly, and after- 
wards forms a lining to the roof of the perivisceral cavity, splitting in the centre to 
enfold the segmental ducts. It continues all the way back to the urinary vesicle, 
diminishing much posteriorly, then dips in between the vesicle and the rectum and 
ceases. In the case of those embryos long retained in the egg, the pigment is abundant. 
The general hue of the fish is a translucent straw-colour, with the exception of the 
blackish pigment just referred to. As the latter increases it is found that dorsally, in 
lateral view, it becomes aggregated largely in two lines above the level of the notochord, 
and gradually reaches the base of the tail. Much of this pigment is doubtless deep seated, 
that is, is developed in the membranes of the brain and spinal cord. 

In the beginning of April the blackish dorsal and ventral bands on the side become con- 
spicuous. Each commences behind the pectoral fin, and envelops the lateral region of the 
body almost to the lower border of the liver, where it is defined by a straight line which 
commences somewhat above the notochord in front, and dips slightly in its course until 
it reaches the anus. It covers the entire alimentary tract, and posteriorly shows 
numerous dorsal digitations. The head is now so closely covered by the stellate pigment- 
spots that it assumes a dull slate-colour, and the same hue characterises the anterior 
region of the body. A double line of pigment runs internally (1) along the tips of the 
neural spines, processes interdigitating with them, and continuing very distinctly almost 
to the end of the notochord; (2) the other passes from a point a little behind the head on 
the surface of the muscle- masses, to cease a little before the other band just described. 
This outer pigment-layer is within the bases of the spines (interspinous bones). Towards 
the end of the month a still further increase takes place in the pigment between the snout 
and the base of the tail, and it extends latterly to the ventral marginal fin. The densest 
region is at the dorsal margin of the abdomen on each side, and stretching from the 
pectorals to the anus. The fish laterally shows a silvery lustre (from the peritoneum), 
with black touches, which form an obliquely striped arrangement (PL XXVII. fig. l). 
Toward the upper region of the abdomen the colour progressively increases, so that about 


the 1st of May the upper arch of the cranial cavity, and a considerable portion of the sides 
superiorly, are covered with continuous pigment. It extends along the spinal cord — en- 
veloping two-thirds of its surface dorsally — while the lower aspect is free from it. The 
abdominal cavity has a continuous coating on its silvery peritoneal surface, and the same 
occurs, as previously noted, round the segmental organs. A dense layer of pigment also 
lies beneath the skin, over the muscles, and posteriorly the upper arch splits to send off a 
layer to the tip of the interspinous bones. The latter layer ceases a little above the bases 
of the V-shaped fin-rays. There is considerable variation in the pigment, some specimens 
towards the end of June, with the yolk-mass completely absorbed, presenting in trans- 
verse section, besides the continuous layers, large isolated pigment-masses here and there, 
breaking the continuity of the layer, both in the subcutaneous and peri-nervous regions. 

The silvery sheen extends a short distance over the yolk-sac. The dorsum of the 
head is deeply pigmented, while the sides are less so. A little below, the lobes of the 
brain are outlined by their blackish pigment. About the middle of May, the con- 
dition of the young fish is shown in the figure before mentioned (PL XXVII. fig. 1). The 
pigment on the dorsum and sides is dull blackish or a uniform dull grey, with black specks. 
The abdomen is silvery and iridescent, as well as dotted with black pigment-corpuscles. The 
pectorals have a little pigment at their bases, and the same is present along the bases of 
both dorsal and ventral marginal fins, the former being definitely banded. The tail is still 
translucent. The iris is iridescent silvery, with black pigment — chiefly visible superiorly. 

When about five inches in length, the dorsal fin has about a dozen black spots at inter- 
vals, the anterior being nearer the free edge than the posterior. The sides are also marked 
by a somewhat reticulated arrangement of spots which, in a few instances, coalesce to form 
bars. The lateral dark patches proceed from those in the dorsal fin, and meet when about 
a third of the body is traversed, and then they form a single or double band to the ventral 
border, which is devoid of them. The increase in the pigment, e.g., at six inches, demon- 
strates how, from the former arrangement, the bold stripes on the sides and dorsal fin of 
the adult are formed. The hue of the latter must render it very much in harmony with 
its surroundings on rough hard ground amidst stones and zoophytes. 

In the young salmon we find many points of contrast at this stage. The pigment 
on the head is much developed on extrusion from the egg. A few black pigment- 
corpuscles occur beneath the cranial tract and along the spinal cord, but they are 
isolated, and often in the abdominal roof only a single one occurs in a transverse section 
of the fish. In this region, also, a line of pigment passes dorsally on each side of the 
median (embryonic) fin. A few, again, occur at the base of the ventral median fin. Both 
cease before reaching the tail. The young salmon at this period is of a translucent pale 
yellowish hue, with a tinge of pink, while a considerable number of pigment-corpuscles 
are present on the head, and a chain of them along the neurochord. The large oil- 
globule is of a dark orange-colour. The pigment very soon becomes denser at the upper 
part of the yolk-sac, as well as over the body of the fish. The indications of the " parr- 
'marks" occur at the third week. A little later (fourth or fifth week) the pigment 


is much increased, especially at first over the opercular region, and other parts of the head. 
The " parr-marks " on the sides are eleven in number, though, from the close proximity 
of several, they do not appear to be so numerous. They are not symmetrical on the sides, 
the fourth being double on the left side, whereas it is the third that is so on the right in 
the specimen examined. Along the dorsum the pigment is arranged in a double chain, 
while the head shows an hour-glass pattern. The black spot on the operculum is well 
marked. The tail laterally and posteriorly exhibits a brownish -pink hue from the presence 
of the reddish pigment-corpuscles. These latter are likewise distributed on the general 
surface of the body and on the dorsal fin. 

About a fortnight later the entire body is covered with pigment-corpuscles, which are 
larger and more closely aggregated at the " parr-marks." Minute glistening granular 
bodies also occur in various parts of the integument. The dorsum anteriorly has a con- 
siderable member of pinkish-red corpuscles, giving it a reddish tint to the naked eye. 
Small corpuscles of the same hue occur in the " fatty " fin, in which the vessels ramify 
very plainly. 

Along the dorsum (behind the pectorals) are seven pigment-masses. The first, imme- 
diately behind the pectorals, is large and distinct. The second, also distinct, is situated 
at the anterior part of the dorsal fin, covering a space in front and a portion on each 
side of the fin. The third has similar relations to the posterior part of the fin, besides 
proceeding some distance behind it. An elongated and less defined mass follows, in front 
of the " fatty " fin, and connected with a smaller patch at the anterior border of the fin, 
which proceeds a short distance on either side. Eight fairly definite lateral bars are 
present on the left side, with a less evident ninth bar at the tail. On the right, likewise 
are nine, the sixth being placed a little behind the (posterior) margin of the dorsal fin, and 
so elongated and large as to appear double or compound. The ventral surface is silvery 
white, and the general colour of the back greenish. Scales are now present on the 
dorsum, and mucus scraped from the surface is found to contain many flat nucleated cells 
(epithelium) of a delicate transparent character, with a large granular nucleus, and one or 
more nucleoli. 

The young salmon, about a month older, shows a little variation in the "parr-marks." 
Thus, in some eleven occur on the left, while nine are present on the right side. The 
dorsal pigment remains much the same as above described. A reddish tint is still observ- 
able at the margin of the fins. The young fishes, however, differ somewhat in size and 
colour, some being quite light, others very dark under the same conditions. In many 
examples a large yellowish patch occurs in the posterior part of the cranial region, while 
in front is a greenish-yellow area. The usual changes in tint, however, are observed on 
removal from a darker to a lighter situation, except in sickly or apparently dying fishes. 
The round glistening bodies composed of transparent granules are still abundant. 

The pigment of the two species mainly considered in the preceding pages offers a few 
interesting features to which brief reference must now be made. 

The young wolf-fish is uniformly tinted, or at least exhibits no bars, in the stages up to 


the length of 1^ inch ; whereas the adult wolf-fish is boldly barred with dark pigment. 
On the other hand, the young of the salmon, as well as of allied species, are just as 
prominently barred in their larval state, as they are uniformly tinted in their adult con- 
dition. We have seen that the young cod is at first boldly speckled, and later is definitely 
tesselated, whereas in its adult state the tints are more or less uniformly arranged. 

The ling (Molva vulgaris), again, after the ochre-yellow colour of the early embryo — a 
colour which extends along the enormous pelvic fins of the subsequent stage — becomes, in 
a more advanced condition, beautifully striped, and still later is barred in a most striking 
manner, while the adult form is almost as uniformly tinted as the cod. The young green 
cod (Gadus virens) is, however, uniformly tinted both in its young and adult condition, 
the larval stage not yet having been described ; and, as far as is known, the haddock and 
whiting (PI. XVII. fig. 12), after the absorption of the yolk, do not exhibit any divergent 
feature in regard to colour. On the other hand, the young gurnard (T. gurnardus) shows 
little that is noteworthy in the pigment of the early stages, but when f inch in length it 
is characterised by the remarkable crescentic disposition of the pigment on the pectoral 
fins. The dragonet (Callionymus) is less beautifully tinted in the young stages than in 
the adult, but it is noteworthy that the ventral surface of the abdomen is blackish in the 
post-larval stage — white in the adult. 

The rocklings (Motella) are distinguished by their remarkably long ventral fins, the 
base being white and the tips black. In the Pleuronectidce there is a tendency to 
transverse rows of blackish spots, as in the turbot (Rhombus maximus), or in an earlier 
stage to dots along the bases of the marginal fins, both dorsal and ventral. 

The Integument. — Few features of interest are found in the integument of other post- 
larval forms, as the scales appear to be late in developing, and the various dermal and 
epidermal strata are not readily distinguished. In some forms, as in Callionymus, ^ inch 
long, and Cyclopterus, large cellular spaces occur from the snout to the tail — developed 
probably in the Malpighian stratum of the integument, and by their increase in size they 
are pushed towards the surface. Their contents are usually clear in stained sections ; but 
in Cyclopterus they are coloured very deeply by Beale's carmine, and in all cases are 
probably glandular. In Cottus, § inch, also very large spherical cells of a similar character 
appear ; and in the post-larval Labrus, T 7 ^ inch in length, they are most numerous over 
the frontal and epiotic regions. 

Serous Spaces. — In the early larval condition of certain forms considerable serous 
spaces occur in the dorsal regions of the head and trunk, but they gradually diminish, 
and lymphatic fluid appears to collect in chambers, often of large capacity, which occur as 
in the post-larval gurnard when -fa inch in length, between the roof of the mouth and the 
cranial floor. A blade-like plate of hyaline tissue is developed in the membrane arching 
over the two supra-oral cavities. Each plate is placed at an angle, and abuts on the 
posterior median process of the rostrum. Again, in the cod, |^ inch in length, a large 
lymphatic chamber occurs behind the urinary bladder, and a network of connective 
tissue occupies the contained space. Around the fore-part of the membranous cranium 

VOL. XXXV. PART III. (NO. 19). 6 X 


in Pleuronectes Jlesus, ^ mcn m length, a space occurs — evidently filled by gelatinous or 
lymphatic matter with widely separated nuclei. 

Pectoral Fins. — In the larvae of the wolf-fish hatched at the time the eggs were obtained 
(viz., January) the movements of the pectoral fins (PL XX. fig. 5) were as active as those 
in the salmon on its escape from the ovum. Their motion within the egg, however, was 
not noticed at this period. As the season advanced these organs showed more vigorous 
motions, and were kept in rapid vibration when swimming. On the 1st of April they pre- 
sented a crenate margin, and before the end of that month attained great development as 
large fan-shaped expansions, but during the stages under observation their axes remained 
more or less vertical, as indeed they are in the adult. A. comparison with the salmon is 
interesting, since, in the wolf-fish, the pectorals remain very large throughout life, whereas 
in the adult salmon they are of moderate dimensions. Viewed laterally in the newly-hatched 
salmon (PL XXII. fig. 4) they are very prominent, rising above the line of the dorsum and 
its median marginal fin. In vertical transverse section their tips far exceed the dorsum, 
and thus they present a great contrast to those of the wolf -fish, which do not at this stage 
reach so high. Their plane is nearly parallel with that of the body, and they are more or 
less rounded in form. On the fifteenth day they show embryonic fin-rays, and distinctly 
droop to the sides, and do not extend so much above the dorsum (PL XXII. fig. 8). 
As development proceeds, they gradually increase in size, and by the rotation of their 
axes they, in the fourth or fifth week, assume a horizontal position (PL XXII. fig. 10). 
They form, indeed, two shark-like organs which, when the fishes are resting on the stones 
at the bottom, are often moved in a reptant fashion, after the manner of Chelonian fore- 
feet. Moreover, the fin of the parr, at a somewhat later stage, becomes less rounded 
(more lanceolate) than in the earlier form. In horizontal sections of post-larval Gadoids, 
£-| inch long, the free fin-plate of cartilage is separated by an interval from a proximal 
(coraco-scapular) plate, internal to which is the hyaline clavicular rod. The interval 
referred to is filled up by deeply stained cellular tissue which forms a thick protruding 

Ventral Fins. — A similar change takes place in the shape of the ventral fins in the 
salmon, from the rounded form of the seventh day salmon (PL XXVIII. fig. 2) to the lan- 
ceolate outline of the adult. These organs (ventral fins) are sometimes used for support 
when resting on the bottom. In the early larva only the embryonic fin-rays are present, but 
at the end of the third or fourth week there is externally a pale margin, with few cells, which 
increase from without inward until a mass of cells — traversed by the fin-rays — appears. 

The ventral fins in marine fishes develop late in larval life, one of the most rudi- 
mentary stages occurring in a post-larval Ammodytes {?), £ inch in length. On the 
flattened ventral surface two angular knobs appear laterally below the posterior hepatic 
region. In transverse section the epidermic evagination exhibits an inner columnar layer, 
with a dense central core, limited internally by the thin peritoneum. A similar appearance 
is presented by the ventral fin-buds in Gastrosteus spinachia at a late larval stage. In 
Pleuronectes Jlesus, 2 \ inch in length, the buds are more advanced and have the form of 


ventrally directed flaps on each side of the diminishing median embryonic fin-membrane. 
In the central core above referred to, a simple bar of cartilage passes downward from a 
point immediately below the peritoneum, at the anterior border of the liver, and ends, 
near the tapering margin of the fin-bud, in a dense aggregation of small cells deeply 
stained in the preparation. In a Gadoid, f inch in length, similar cartilaginous plates 
appear, not, however, in bold projecting flaps of the integument, but in flattened hori- 
zontal ridges ; this difference in position being due, no doubt, to the flatter and more 
obtuse character of the ventral surface in the post-larval Gadoid. Posterior to the paired 
cartilaginous plates, which have an upper and a lower muscular mass, an expansion of 
the integument forms a membranous fin supported by four or more hyaline rays. The 
fin-rays at this time consist of paired hyaline rods, semilunar in transverse section, each 
pair enclosing a strand of dermal tissue. A still later stage is seen in the post-larval 
goby, ^ inch long, in which the two basal skeletal rods of the developing ventral fins 
approximate, and are united posteriorly, forming an angle, with the apex directed back- 
ward. Thus early is the union of the ventral fins effected which results in the disk -like 
structure characteristic of the adult. The liver lies immediately over the developing 
ventral fins, which are slightly anterior to the position they subsequently occupy. 

Median Unpaired Fins. — In the larval wolf-fish the marginal fin-membrane 
commences behind the head in a position similar to that in the salmon, and extends 
all round to the anus, and again forward in front of it to the yolk-sac. Proportionally 
it is much less developed than in the salmon, and while the changes in its outline in the 
latter are complex, those in the wolf-fish are less so. Moreover, whereas the marginal 
portion of the tail of the salmon does not increase much at the period of absorption of 
the embryonic fin, the contrary is the case in the wolf-fish, whose caudal expansion 
attains large dimensions as the fin of the body diminishes. Whether this is altogether 
due to the increase of the marginal web of the tail, or absorption in the other parts, is 
doubtful. Probably both causes are concerned. The embryonic fin-rays appear first in 
the caudal region, and afterwards in the anterior region. As the larval fish increases in 
size, the marginal fin remains stationary, its further apparent diminution being due 
merely to the general increase in the bulk of the body. It, however, increases in 
thickness, and spinous rays are developed during the second month. On completing the 
larval phase, i.e., on the absorption of the yolk in the beginning of June, the number of 
dorsal spines is seventy-one; the first pair are separate, and the second diverge at the 
tip, while the last is single and small. The ventral spines are forty -four in number, and 
the last likewise is single and small. 

In the salmon the marginal fin (PI. XXII. fig. 4) shows, on emergence, considerable 
differentiation. Thus the first dorsal is well marked, though it has only the granular 
structure characteristic of the rest of the fin. The adipose fin is indicated by the eminence 
situated between the former and the caudal, which is somewhat lobate, and generally 
shows a slight notch above the tip of the notochord, marking, apparently, the homologue 
of the embryonic tail-fin. The anal is distinguished by an elevation posterior to the anus, 


and a similar prominence (pre-anal) corresponding to no structure in the adult fish, and 
not comparable to the first anal of the Gadidse, occurs between the ventral and the anus.* 
When a week old, these portions, which correspond to the regions of the permanent fins, 
are denser, the other parts being very thin, and apparently undergoing absorption. 
The embryonic rays are now distinct. Meanwhile the blood-vessels begin to ramify- 
in the " fatty " fin (PL XXVIII. fig. 3), and the capillaries in front and behind the 
primary series are on the ninth day increased; they soon, indeed, extend throughout the 
entire length of the fin. Several indications of true fin-rays occur in the first dorsal, and 
at the end of a fortnight the embryonic marginal fin has, to the naked eye, nearly 
disappeared, so that the permanent fins are more boldly marked. Pigment appears in 
both dorsal fins at the same period ; while pale capillaries ramify in the anal fin, and 
stretch nearly to the tip. They probably also develop in the dorsal and ventral ; but 
they were not seen. The marginal (embryonic) fin is now almost absorbed, except in 
the interval between the ventral fin and the anus. 

Between the fourth and fifth weeks, the dorsal and anal fins show the cartilaginous 
rays, while the membranous parts between them are widened and coloured by numerous 
yellow and brownish pigment-corpuscles. Thirteen rays occur in the dorsal, and the 
same number in the anal fin, and the interspinous elements produce wavy marks beyond 
the muscle-plates. Both fins have crenate borders, as in the tail at this period ; while 
the adipose fin presents a fibrillated aspect, and has a network of fine blood-vessels. All 
the fins are proportionally larger than in the adult, as observed in the outline of an 
average specimen (PI. XXII. figs. 10 and 11). In minute structure the dorsal and other 
fins already present most of the characters of the adult. Thus, at the anterior part of 
the dorsal, are two narrower and shorter rays, the first a simple spike, the second con- 
sisting of two which form a loop. At the base of the larger rays is a projecting median 
point, and the terminal process is long, and almost unciform. The pigment is chiefly 
placed on the cartilaginous rays. 

In considering the condition of the median fin-rays of these young fishes, it will be 
observed that on the thirteenth or fourteenth day the dorsal of the soft-rayed salmon 
corresponds nearly with the condition in the adult, but in the anal fin the number of rays on 
the tenth or the twelfth day, viz., thirteen, is in excess of the number in the adult. In this 
respect, however, it is doubtful how far authors in numbering the rays have anatomically 
examined the parts, or how much they have depended on external appearances. On the 
other hand, the osseous rays in the dorsal of the young wolf-fish seem, if Day be right, 
to be fewer than in the adult, viz., 71 as compared with 72 or 74 (Day), while the rays 
in the anal present a similar condition, viz., 44 in the young as against 45 to 46 in the 
adult. An examination, however, of the skeleton of a fine adult in the University 
Museum here shows that the number of the dorsal fin-rays exactly corresponds with that 
of the young forms just mentioned, and so with the ventral. There may be variation ; 

* The outlines of the fins of the young salmon seem to differ considerably from those of the larval Lochleven 
trout, as shown by Mr J. T. Cunningham, Trans. Boy. Soc. Edin., vol. xxxiii. pi. i. fig. 4. 


but it is remarkable that so little exists. Mr Day * had, therefore, certain grounds for 
stating that fin-rays do not materially increase with age. 

The condition of the growing rays and of the interspinous elements in certain other 
post-larval Teleosteans is interesting. Thus, in a section of the caudal trunk of a 
gurnard, T 5 2 inch long, four series of interspinous cartilages are developed in the 
connective-tissue strand which extends upwards from the neural arch. The neural spine 
is the proximal element, and has the form of a rounded nodule clothed on each side by a 
plate of hard hyaline tissue. The third nodule in the series is large and irregular, and 
like the second presents a cartilaginous structure only ; but the highest nodule, lying at 
the base of the 2nd dorsal fin, exhibits on each side a horizontal hyaline plate passing 
outwards parallel to the plane of the flattened dorsum. The fin-rays are paired rods of 
hyaline tissue, and at this time each rod is separated from its fellow by intervening 
tissue. Each of the cartilaginous elements just described has its special muscular strand 
on each side, and it is possible that they may at a later stage unite to form one javelin- 
like neural spine. Further forward, indeed, the neural spine does develop as a single rod 
of cartilage surmounted by a nodule of the same tissue which is trifid superiorly. In 
Cottus quadricornis the neural spine in the mid-trunk region is cartilaginous, with one 
superior (interspinous) element ; but the neural arch itself is formed by two dense 
hyaline arms which grow out from the perichordal ring of the same tissue. In the post- 
larval goby (Gobius ruthensparri) the interspinous element appears before any neural 
arch or spine is developed, and it has the form of a rounded nodule at the summit of the 
connective-tissue continuous with the perichordal sheath. In the young wrasse, ■£$ inch 
in length, no supra-neural cartilage appears at all in the region of the dorsal fins. The 
fin-rays are, as usual, formed by the union of paired hyaline rods, having a semilunar 
form in transverse section — the concave surfaces being opposed, and the neural spine, as 
will be described later, is formed of hyaline matter. 

On escaping from the egg the tail of Anarrhichas is more or less lobulated, though 
in many it is lanceolate. A distinct constriction (PL XX. fig. l) occurs in front of the 
organ, which then gradually widens out — the dilatation being more marked inferiorly 
than superiorly. The notochord goes straight backward from the nuchal curve to the 
commencement of the caudal region proper, and then tapers to the termination, the axis 
of this part being in the same line as that in front, viz., horizontal. In many views, 
in newly hatched forms (January), a slight fold appears at the margin of the caudal fin, 
in a line continuous with the notochord ; but whether this is a definite structure or not 
is uncertain. All that can be said is that the appearances indicated in the figure 
(PL XX. fig. 1) were frequent, and recalled the notch above the tip of the notochord so 
familiar in the larval salmon. Structurally the tail, at this stage, presents a minutely 
cellulo-granular appearance (due to the cutaneous elements), most marked in the thicker 
central region, and becoming translucent towards the margin. The embryonic fin-rays, 

* [It is with deep regret that the authors have just received intimation of the death of this experienced and meri- 
torious worker in the field of Ichthyology — both British and foreign.] 


like fine fibres, radiate over the whole area of the organ. The notochord transfixes the 
tail considerably above the median line, leaving a much larger proportion of the lobe 
beneath than above, and into this the blood-vessels pass. A little posterior to the 
marked caudal diminution of the notochord the neurochord abruptly narrows, and a 
delicate continuation (PI. XX. figs. 1 and 3) proceeds to the tip of the former. 

In the course of the succeeding month (February) the dorsal bend of the noto- 
chord commences, so that on the 17th a considerable change in outline has occurred. 
The upper border, especially along the posterior line, becomes prominent, while the 
inferior is less so, and the hind edge assumes a broad blunt outline (PL XX. fig. 3). 
It is evident that the inferior lobe of the tail in the early stage (PL XX. fig. 1) now 
becomes more or less the posterior, a change, perhaps, partly due to the terminal and 
upward curve of the notochord. The latter at this stage is much less marked than in the 
salmon of the first da)^. The appearance of the hypural elements (PL XXVII. fig. 2), as 
already indicated, probably aids in the transformation. The notochord is still less curved 
than in the salmon of the first day, and the posterior hypural margin slopes downward 
and forward. The hypural (cartilaginous) element most anterior (hue) is quadrate, and 
devoid of the notch seen by Professor Huxley in that of Gastrosteus* Its longest side 
is directed inferiorly and posteriorly. The posterior or superior hypural cartilage is 
triangular in outline, its longest side being applied to the under surface of the ascending 
notochord, which projects about half the length of this side beyond it. The bases of five 
or six caudal rays rest on the larger hypural, and a somewhat smaller number on the 
upper. They may be estimated at twelve in the earlier stage (March). In the following 
month (April) the further curvature of the notochord upward is accompanied by a 
tilting of the posterior edges of the hypurals into a nearly vertical position, and the 
greatly elongated vessels now run straight outwards along the rays. The posterior 
margin of the caudal fin has also become conspicuously crenate, and at this stage (PL 
XXII. fig. 2) the inferior margin is more rounded than the superior, which ends after 
a straight course somewhat abruptly in a crenation. In front of the tail, ventrally, a 
slight inflection of the marginal fin occurs. The notochordal sheath now shows serial 
constrictions indicating the separation of the centra. 

In the next stage (PL XXVII. fig. 3) the tail is considerably elongated, and its 
vertical diameter is diminished. The notochord is less in proportion to the other parts, 
while the anterior, or inferior, hypural has increased in length, and shows a distinct 
upward curvature at the base. 

There are upwards of twenty caudal rays, i.e., about twenty-four, a larger number than 
is present in the adult. Day records the number in the adult at fifteen to eighteen, 
while in the St Andrews University Museum three specimens each possess twenty-one. 

The fin-rays show three vertical rows of articulations, and they spread out distally, 
and terminate in fine fibres, like those of the embryonic fin. The marginal crenations, 
posteriorly, are now so disposed that they correspond with the expanded ends just 

* Quart. Jour. Micr. Soc, 1859, p. 40, pi. iii. fig. 1. 


mentioned. A further stage is shown in the coloured figure of the young fish about the 
middle of May (PL XXVII. fig. 1). In this the upward direction of the dorsal border 
of the tail is evident, but the organ is perhaps less elongated (antero-posteriorly) than 
in the previous stage. 

The difference between the tail of the larval fish and that of the adult is obvious, 
for in the latter it is somewhat dwarfed by the great development of the median fins — 
dorsal and ventral — and its antero-posterior diameter is considerably less than the depth, 
for instance, of the dorsal ; whereas, in the young stage, its long diameter much exceeds 
the depth of the latter, and both the dorsal and the ventral diminish in front of it so 
much that it is very prominent. Probably the condition of the tail has relation to the 
more active pelagic habits of the larval animal at this period. 

The development of the caudal region in the salmon was observed at the fortieth day 
after fertilisation. The tip at this stage is very transparent. The termination of the 
notochord is gently curved upward and ends in a somewhat blunt point, a short distance 
within the free border, the cells ceasing before the tip is reached. The notochordal 
sheath is well marked. Above the notochord the neurochord is about the same breadth. 
The embryonic fin-rays stretch outward as in the marine Teleosteans. Thus the embryo 
has reached at this time a more advanced condition in regard to the notochord than the 
larval wolf-fish in the early months. 

In the newly hatched salmon, again, the tail (PL XXVIII. fig. 1) has attained a 
degree of development comparable with that of the wolf-fish at the stage shown in 
PL XXVII. fig. 2, for the true fin-rays have made their appearance in addition to the 
transient embryonic rays, though the great increase in cellulo-granular tissue obscures 
both. On the tenth day the fin-rays number about twenty, and they abut on the 
hypural cartilages. They are split a short distance from their origin, and blend, in 
ordinary views, in the embryonic fibres at the tip and the cellular stroma of the organ. 

The hypural elements have also increased considerably, and show a cartilaginous 
structure, forming a clear space behind the notochord. The notch remains in the margin 
beyond the tip of the chorda, indicating perhaps the dorsal and ventral lobes ; but the 
tail is narrower, and the posterior margin is less curved. 

When a fortnight old, lines of pigment occur in the tail, along the fin-rays, leaving 
clear intermediate spaces. The arteries run along the latter, and the veins return by the 
dark lines. There is now no notch superiorly to indicate the tip of the notochord, and 
the free margin is somewhat undulated. The upper border of the tail is more prominent 
than the corresponding lower margin, just as we see in Anarrhichas. The segmentation 
of the fin-rays produces wavy lines in the tail. Already the curved portion of the noto- 
chord is diminishing in proportion to the rest of the tail, and is now much obscured by 
pigment. Between the fourth and fifth weeks the articulations of the cartilaginous 
caudal rays produce three wavy vertical lines, with their convexity directed backward. 
The rays are also more distinct, and for the greater part of their length are free from 
pigment, the base alone possessing it. The posterior margin is uneven — from crenations, 


which correspond to the fin-rays. Numerous brown pigment-corpuscles exist in the 
intervening membrane. From the curved portion of the notochord the hypurals (about 
six in number) project backward, besides an opaque mass at the tip of the organ, which 
may be a rudiment of one, though no cartilage-cells can be made out. The fourth ray is 
much broader than the others, and has the form of a long bent spatula. On comparing 
the salmon of this period with Lereboullet's trout at a similar stage,* the following 
differences appear : — The fifth hypural from the tip is large and broad in the trout, instead 
of the fourth. The fin-rays in the trout are fibrillar throughout, and have no articula- 
tions ; whereas in the salmon they are only fibrillar along the terminal third, and have 
the articulations described above. The development of the trout must therefore be slower, 
though in Lereboullet's figure the hypurals are well formed ; or perhaps the figure itself 
is deficient. The condition of the tip of the notochord would seem to show that the stage 
figured is really earlier. 

The pigment in the tail subsequently increases, as also do the fin-rays, which at this 
stage amount to twenty-three or twenty-four ; whereas Day gives those of the adult at 
nineteen. Four vertical lines indicate the articulations of the rays — the first or anterior 
being most curved. 

The caudal region just described, and that of the wolf-fish, offer several interesting 
points of contrast, the most noteworthy of which is, perhaps, the great complexity of the 
circulation in the former on hatching, and the conspicuous curvature of the end of the 
notochord. Besides, the divergent condition of the hypural elements is striking, the 
salmon having five or six narrow elongated hypurals along the upward flexure, while the 
wolf -fish shows only the two broad plates, as in GastrosteusA It is probable that this 
diminution of the hypurals coincides with a reduction in the number of vertebrae in the 
curved portion of the notochord, a supposition borne out by appearances in the salmon 
— for indications of six or seven constrictions are visible in that species in the region of 
the hypurals, that is below the atrophied terminal process, which is lodged between the 
fin-rays. On the other hand, only two are seen in the wolf-fish, and though not shown 
by Huxley in his figure of the stickleback, similar features were probably present. The 
salmon, at any rate, would thus appear to be more heterocercal than the wolf-fish. 

Notochord. — The notochord in the newly hatched A narrhichas (in January) is similar 
externally to that of the salmon — that is, it has the usual pointed anterior end, with the 
downward curvature toward the oesophagus. The structure of the chorda also corresponds, 
though on the whole in the wolf-fish it seems more firm, since it preserves its circular out- 
line (in section), and its cells appear to be smaller. The notochord of the salmon is prone 
to collapse, a feature, however, that may be due to the preparations. Posteriorly, again, 
a decided difference exists, since the notochord in the wolf-fish is at this stage quite 
straight. The straight notochord of the young Clupeoid, ^ inch in length, is remarkable 
for the regular serial arrangement of the large chambers extending from end to end. 
The post-larval condition is thus like that of the earlier larva in this feature ; but the 

* Ann. des. Sci. Nat., t. xvi. p. 184, pi. iii. fig. 42. t Huxley, op. cit. 


notochord has increased in diameter, and in transverse section is disproportionately large 
as compared with the slender character of the trunk in this species. The partitions 
stain deeply, but present no noteworthy histological features. A Clupeoid, double the 
length of the example just referred to, shows little alteration in the structure of the 

During development Balfour states (No. 11, p. 456), " that most of the protoplasm 
with the nuclei is caused to pass to the periphery, where it forms a special nucleated layer, 
sometimes divided into special epithelial-like cells " of which he gives a figure in the case 
of the salmon, " while in the meshes of the reticulum a few nuclei surrounded by a little 
protoplasm still remain." Ryder mentions that he has not been able to see nuclei in the 
cells of the notochord ; # but such appear to be present in Anarrhichas, since small gran- 
ular areas, distinctly stained, occur frequently throughout. They are less definite than 
the nuclei of the newly-hatched salmon, which form large rounded or ovate bodies with 
granular contents, and nucleoli in the majority of the cells (PL XXVI. fig. 5). Moreover, 
the cellular rim within the sheath, as Gegenbaur shows in the salmon,t is well seen in 
longitudinal vertical section in the wolf-fish, since it separates from the sheath as a deeply 
stained layer of cells of some thickness. In the post-larval Coitus scorpius (f inch long) 
a single layer of large nucleated cells lines the chordal sheath, now firm from the deposi- 
tion of hyaline matter. The central mesh work of chordal cells appears to be diminishing 
in diameter, and along with the cell-layer just named is separated from the hard sheath. 
A similar grouping of nucleated cells is very marked within the chordal sheath of T. gur- 
nardus, £ inch in length ; indeed, there are several features of interest at this stage of 
post-larval life, for external to the round nucleated cells J just mentioned, with their 
definite nucleus and clear cell-contents, is a layer of very much flattened cells. The 
large cellular spaces of the notochord have thin walls irregularly folded, and the more 
centrally situated chambers are more spacious than those outside. This condition is also 
well marked in the post-larval wrasse, about ^ an inch in length, the larger cells being 
central. When ^ of an inch in length the notochord of the post-larval Gadoid shows 
indications of vertebral divisions, dense transverse aggregations of nucleated tissue, 
flattened cells, and amorphous notochordal plasma, forming serial rings on the inner sur- 
face of the chordal sheath. As these ridges grow they rupture the hyaline sheath. 

In the beginning of March the chief change in the notochord of the wolf-fish is the 
increase in the size of some of the median cells, those next the circumference being 
smaller. The nuclei, with their nucleoli, are also very distinct in many. Little altera- 
tion occurs in the general arrangement of the notochordal cells in May (PL XXVI. 
fig. 4), and though the development of the vertebral elements has made considerable 
progress, yet the cells show little or no modification of importance on the 20th of June. 

* He writes — " In just hatched embryos of several genera I have as yet failed to discover any trace of nuclei in 
those portions of their walls which extend into the body of the chorda" (Rep. U. S. Fish. Commiss., 1882, p. 511). 
t Elements of Comp. Anat., translated by Prof. Jeffrey Bell, p. 427, fig. 221 . 
% In prepared sections these cells recall precisely the appearance of the early blood-corpuscles in Teleosteans. 

VOL. XXXV. PART III. (NO. 19). 6 Y 


There is thus much less complexity in the stages just described than shown by Balfour 
and Parker in the notochord of the Ganoids.* 

Skull. — Much has been done in regard to the development of the cranium of 
Teleosteans, and the comparatively recent memoir of Mr PARKERt specially treats of the 
skull of the salmon, so that it will be necessary to give only a brief account in these pages. 

On escaping from the ovum in January, the cranium of the wolf-fish is in a very 
rudimentary condition. The greater part of the vault is covered by a thin layer — the 
external cellular integument with a membranous layer beneath, the former showing a 
considerable thickening above the prominent ocular region similar to the tissue — con- 
sisting of pulpy columnar cells — in Gadoid larvae -^ inch long. In vertical section 
the first skeletal elements in this region are the anterior ends of the trabecules (PI. 
XXIII. fig. 1, and PI. XXIV. figs. 5 and 6, tr), which seem to have united in front, as in 
the salmon of the first day, and form a kind of ridge, with a superior convexity. They 
extend downward and inward, leaving a space behind the pituitary body, and appear 
to merge in the parachordals which lie on each side of the notochord. The notochord at 
its commencement abuts, in fact, on the cartilaginous plates just mentioned. The inner 
ends slightly curve upward, and do not appear to touch. The parachordals in the 
post-larval Pleuronectid, fa of an inch long, furnish a great contrast to this condition, for 
their inner edges have coalesced, and form a dense plate of cartilage — into a cylindrical 
cavity in which the anterior end of the notochord passes. From this dense central plate 
thus pierced by the oral end of the chorda, two thin plates pass and unite with the 
otocystic cartilage on each side. The basilar plate now forms a complete floor in the 
posterior cranial region. Even more marked is the united condition of the parachordals 
in the young Clupeoid, f inch long, at the point where the notochord passes into the 
cranium — the coalesced part having, in transverse section, the form of a massive oblong 
element penetrated centrally by the notochord. Posteriorly, as the diameter of the 
notochord increases, the cartilaginous investing mass diminishes, until between the ears 
it is represented merely by four angular nodules of cartilage, two at the upper and 
two at the under side of the chorda. In the post-larval goby, -fa inch long, the 
parachordals unite, but they form a comparatively thin, flattened, basilar plate, into which 
the chorda passes, and their lateral extensions unite with the otocysts and continue 
upwards over the hind brain — the posterior part of the chondro-cranium being, in fact, 
now wholly cartilaginous, and this complete tube of thin cartilage continues into the 
occipital region, and encloses the medulla oblongata. The small Gadoids, ^ inch long, 
show a similar condition of the posterior floor of the skull, the thin cartilaginous 
parachordals uniting with the floor of the otocyst on each side ; but the roof is still 
membranous above the fourth ventricle. There is a remarkable development of black 
pigment in the lining membrane of the otocystic cartilages — the corpuscles being situated 

* The development of the vetebral elements of Teleosteans at St Andrews has been undertaken by Prof. D. J. 
Cunningham, of Dublin. 

t Phil. Trans., vol. clxiii., 1873, pp. 112-145, plates i.-v. 


below the epithelial layer and the sensory cushions, and internal to the auditory ganglion, 
beneath which, and over the upper surface of the parachordal cartilages, it extends as a 
well-marked stratum. In the cod, T \ inch long, the medulla is arched over by a deeply 
angular supraoccipital cartilage, having the form of an inverted V, and upon its sloping 
surface on each side a flat sheet of muscle passes. A little behind, these plates in the 
wolf-fish join a cartilaginous lamina, which bends upward and outward to form the floor 
of the auditory chamber on each side. In the median line above the tip of the notochord, 
and extending in front of it, is the infundibulum, but it disappears in the line of the 
laminae forming the otocystic floors. After its disappearance the cartilaginous plates 
(parachordals) greatly increase in bulk, so that the notochord between them is strongly 
buttressed. Posteriorly, however, the density of the plates diminishes, and they spread 
outward, extending under the auditory sacs. Two thickenings occur here, one externally 
and another internally. With the former is fused the hyomandibular element. The 
thin band connecting the two parts mentioned gives way as one of the semicircular canals 
(probably the horizontal) comes in the line of section, and only connective tissue there 
unites them. They seem to be closely connected behind, though this could not be accu- 
rately made out. A similar disconnection is noticeable in the post-larval Gadoid, |^ inch 
in length, — the basilar plate sending up ex-occipital elements to the upper posterior face of 
the otocysts, the cartilage of both being continuous, while below is a well-marked interval. 

The tendency of the auditory capsule in Anarrhichas to sink downward is occasioned 
by the disappearance of the cartilaginous floor just described. The cartilaginous support 
of the notochord in transverse section now assumes the form of a broadly lanceolate process 
seated upon each side, the outer end abutting on the wall of the descending auditory 
sac, while the inner envelops the notochord. The cartilage curves upward fully half- 
way round the medulla, leaving the lower part of the notochord free. It then disappears, 
and the notochord, which has not at this point attained its full dimensions, is surrounded 
by perichordal connective tissue (by which also the side of the neurochord is clothed) 
supported by the muscle-plates. 

An evident difference between the foregoing arrangement, and that in the salmon one 
day old, is the fact that the trabeculse, while united and large anteriorly, diminish on their 
way backward, thin off in the middle, and then separate in the ocular region, becoming 
still less as well as more widely separated as they proceed backward. In the infundibular 
region, they again increase in size, but are separated by a considerable interval, and, 
shortly after the commencement of the auditory capsules, the anterior end of which is 
cartilaginous, a plate passes from them, with a slight obliquity upward and outward, and 
joins the cartilaginous boundary confined in this region to the floor and outer wall. The 
curvature of this floor quite differs from that in Anarrhichas, for, like Gobius and other 
forms, it is nearly flat. The hyomandibular, forming a buttress at this point, is narrow 
superiorly and more dilated inferiorly (more truly clavate) than in the wolf-fish. The 
pointed anterior end of the notochord now intervenes between the plates, which, however, 
do not touch it. Each plate somewhat behind this region becomes stouter ; but the 


lateral extension joining the ventral auditory plate diminishes, while the auditory capsule 
passes upward, and forms a dense structure superiorly — below the anterior semicircular 
canal. Then the basilar plates separate from the auditory capsules, and form, in section, 
an elliptical rod at each side, increasing in bulk further back, and again sending out a 
lateral plate to join the floor of the auditory capsule, and finally separating from it. 
Thereafter, each lengthens upward and outward, and joins the continuous cartilaginous 
mass bounding the auditory capsule posteriorly. Behind the latter capsule the cartilage 
passes upward from the sides of the neurochord, diminishing as it goes, nearly to the 
middle line of the dorsum. In sections further back we see it diminish to a lateral plate, 
pointed above, broad where it clasps the notochord, then broad above and pointed below, 
and still further back it disappears. There are material differences, therefore, in the car- 
tilaginous elements of the two forms, one of the most marked being the larger size of the 
ear-capsules, and the deficiency of cartilage posteriorly in Anarrhichas, and the greater 
bulk of the cartilage at each side of the notochord behind the auditory region in the same 
form. The cartilage is evidently much more firm than in the salmon, and instead of 
collapsing the notochord, as before mentioned, remains circular. The cartilages of the 
posterior part of the cranium are, however, of greater complexity in the salmon. Further, 
in the latter form a slender bar represents the palato -pterygoid ; but in the wolf-fish no 
trace of it is present. Its position in the salmon is very different, for, as Mr Parker 
points out, it (his palatine) lies under the eye on each side, and each widely diverges from 
the other, whereas in Anarrhichas these bars are much nearer the trabeculse. 

Early in February (about the 10th) the pterygo-quadrate bar appears in Anarrhichas. 
In transverse section it has the form of a small cartilaginous rod in the anterior region of 
the snout (PL XXIII. fig. 1), just beneath the outer edge of the trabeculse, which form a 
ridge in section. The bar seems to increase from before backward, and on 16th March the 
trabecular floor has gained so much in breadth, in the mid-region, that it lies on each side 
within the margin of the said floor inferiorly. Behind this, however, the trabeculse form 
a narrow ridge, and the pterygo-quadrate bar passes outward on each side, and assumes 
an ovoid form in the subcutaneous tissue on the roof of the mouth. It is visible at this 
stage as far back as the pituitary body. 

As development proceeds the anterior end of the trabeculse becomes enlarged, and 
extends upwards in the olfactory region. This is distinctly seen on 16th March (PL 
XXIII. fig. 3). 

On 6th April, a considerable vertical extension of the trabeculse — most conspicuous a 
little behind the tip of the snout — has taken place. In transverse section it presents a 
somewhat hastate outline — broad and bluntly pointed below (with a median notch) con- 
stricted above this, and then slightly dilating upward. The united trabeculse do not yet 
come in contact with the rounded anterior ends of the pterygo-quadrate bar, but after 
reduction in depth and assuming the shape of a broad inverted V, they reach the brain, and 
become continuous with a broad bar of cartilage passing upward to the dorsum on each side 
on the inner border of the eye. Beneath the anterior brain-mass are the crucial muscles, 


and immediately behind the trabecule assume more of their earlier appearance, but with a 
median septum projecting superiorly. In T. gurnardus, -£% inch in length, they send out 
a lateral expansion — a pre-orbital horizontal plate, but further forward they continue in 
the usual cylindrical form. Between the rod-like portion, which forms the main part of 
each trabecular element, and the lateral plate, a region devoid of cartilage-cells passes, the 
significance of which is not easily understood. As the ridge-like shape of the septum is by 
and by more marked in Anarrhichas, and as the cartilage diminishes, the median septum 
becomes a fibroid streak. The chief difference in the shape of the ridge-like bar of the 
trabeculse is the slightly more bulbous condition of the outer edge in transverse section. 

Before the infundibulum is reached, the trabeculse become flat, and then separate, 
the bars in this case sloping from below upward and outward, and between them lies a 
firm plate of hyaline hard tissue, thickest in the middle and bevelled off as it approaches 
the trabeculse. In front of the infundibulum a number of muscular bands pass toward 
the eyes in the middle line, and two blood-vessels soon after rest on the floor. At the 
infundibulum the hyaline floor diminishes, and the blood-vessels become more lateral in 
position. Behind the infundibulum the trabecules widen out and join the basilar plate 
forming the floor of the cranium, where the notochord commences. Thus they do not 
again unite. A further complication occurs about the beginning of May, when two 
slightly converging vertical bars of hyaline ossific tissue appear at the anterior ends of 
the trabecules — passing upward from the bases of the teeth, one dental sac being, indeed, 
placed between the limbs of the processes. The cartilage at this time is divided into an 
upper and a lower region, the latter again being vertically subdivided. The whole nasal 
region is cartilaginous, and further ossifications at the bases of the teeth are present. A 
plate of the same hyaline tissue appears in the free flap at the side of the mouth, and 
seems to represent the maxillary. 

About the end of March a nasal cartilage develops behind the olfactory sacs, being, in 
fact, a superior process (fronto-nasal) of the trabeculse at their anterior ends. A second bar 
of cartilage, the anterior end of the pterygo-quadrate, projects outward in horizontal sections, 
towards the outer angle of the truncated muzzle on each side — at a somewhat lower level. 

The fronto-nasal plate in the post-larval Gadoid, ^ inch in length, is very broad and 
massive, as indeed it is when the fish measures T 5 ^ inch in length, but there now lie on 
each side externally two slender pre-frontal bars. The frontal rudiment appears in the 
young Clupeoid as a plate of cartilage passing transversely over the pineal region, and the 
fronto-nasal process below is flattened and less massive than in the Gadoids. In the 
herring, the frontal rods, passing superiorly between the eyes, broaden out before ter- 
minating over the cerebral lobes. Parietal rudiments appear in the gurnard, T 5 ^ inch 
long, and a sharply pointed hyaline spine, firmly fixed to a horizontal base of the same 
dense tissue, occurs over these paired elements. When T 9 2 inch long the pre-frontal 
cartilages appear over the eye, outside the sclerotic cup, while below each eye are the 
infraorbital elements. The latter are well developed in the Gadoid T 5 ^ inch in length. 
In Callionymus, ^ inch long, the suborbital series arise as hyaline scales. In the 


Gadoid of the size just specified, two hyaline plates pass obliquely on each side of the 
snout — separated by a small interval from the maxillary rods and external to the 
fronto-nasal cartilage. The maxillary elements, which at first are rod-like, become flattened 
at this stage, and may appear, e.g. in Pleuronectes Jlesus, T \ inch long, as two lateral 
hyaline scales. The parethmoids are more median, and lie as two cartilaginous cylinders 
on each side of the stout ethmoidal (rostral) cartilaginous prominence. Above, the two 
optic cups are well advanced, and are also cartilaginous. Beneath the central para- 
sphenoidal bar in T. gurnardus, ^ 9 ¥ inch long, a hyaline plate is formed, possibly the 
vomerine plate. In Labrus, ^ inch long, however, is a similar median plate of hyaline 
tissue, but it is superior to the parasphenoid, and lies immediately under the point where 
the optic nerves cross. In the goby, as Pouchet long ago noted, " the maxillary elements 
appear in cartilage, and, when -^ inch in length, develop a superior crest by a " kind of 
vegetation" of the cells (No. 119, p. 298), while further back an outgrowth in the form 
of a horizontal bar probably represents the pterygoid. In transverse section, therefore, 
two bars are present, an upper or pterygoid and a lower or maxillary bar. They have the 
same character, but are somewhat lengthened in a form ^ inch in length. 

The mandible of the young wolf-fish, on emerging in January, presents a less advanced 
condition than in the salmon. It forms a short conical process with a high cutaneous 
flap on each side. Though its cartilaginous and muscular elements are partly developed, 
it remains quite motionless, a feature doubtless connected with the widely-open mouth. 
Meckel's cartilage forms on each side a long curved bar, which slightly dilates at the 
anterior end, and terminates in a rounded portion — approaching that of its fellow in the 
middle line. The mandibular cartilage in Callionymus, ^ inch long, is clothed on the 
ventral surface with dense hyaline matter, which thins out on the sides of the mandible. 
Outside the cartilage a separate hyaline scale exists, apparently developed in a deeply 
stained mandibular fold of the integument. In the gurnard, ^ inch long, and in the cod, 
T \ inch long, a similar extra-mandibular bar appears. It is very massive in the Gadoid, 
^ inch in length, as well as very prominent in section, since it and the surrounding tissue 
of the flap stain deeply. An element is seen above Meckel's cartilage in Callionymus, 
^ inch in length, probably the symplectic. Posteriorly in Anarrhichas, the mandible 
gradually dilates, and at the articular region forms a slight hollow for the rounded end of 
the quadrate, the cartilage being continued further back as an angular plate, somewhat 
broadly lanceolate in outline. The quadrate seems to be a narrow bar supported behind 
by the pointed end of the symplectic — continuous with the hyomandibular cartilage. 
Along with other changes, the mandible becomes much lengthened in March. 

At the seventh day in Anarrhichas, little elevations appear in the jaw, and towards the 
end of the month one or two simple conical teeth, similar to those in the salmon, present 
themselves in a line anteriorly. These increase in number, still keeping in linear order 
in April, though the yolk is of considerable size. In the free condition the salmon 
feeds readily in this state, the most abundant form found in the stomach being Cyclops. 
Thus both fresh-water and marine larval fishes feed upon similar food. The teeth 


increase both in number and size at the sixth week, large recurved teeth appearing on 
the hyoid, while the pharyngeal cartilages are similarly studded. 

Both wolf-fish and salmon, on hatching, are, however, devoid of teeth ; but develop- 
mental changes seem to be more rapid in the latter than in the former, the cartilages appa- 
rently being more actively plastic, and the advanced stage at which exclusion from the egg 
takes place afford greater time for maturity. Thus at the seventh day Meckel's cartilages 
are separated by a cellular band in front, while they are less rounded than in Anarrhichas. 
A short distance behind is a line marking off the articular piece. Four teeth are visible, 
symmetrically placed and developed from the mucous membrane in the usual manner. 
The first presents a glassy coat of ossific matter, forming the point and sides ; beneath 
this a narrow layer of dentine occurs, while the centre contains the cellulo-granular 
pulp continuous with the oral mucous membrane. It is placed behind the symphysis, 
and is slightly inclined backward ; the second is larger and stronger, with a more decided 
backward curve, while the third is a straight tooth about the size of the first ; the fourth 
is similar, but shorter. In the upper jaw are ten teeth ; but all that can be said is that 
four lay in proximity to the maxillae, two (vomerine ?) intermediate, and two at a greater 

Pleuronectes Jlesus, when j^ i ncn ' m length, has numerous teeth in both the upper and 
lower jaws. They pierce the oral mucous membrane as acutely pointed hyaline structures ; 
the mandibular cartilage at this time being completely encased in hyaline tissue, and 
sending down from its inner ventral margin a sharp crest of the same hard substance, 
whi]e the teeth occupy a corresponding position on the inner margin superiorly, and have 
their points directed inward. Numerous papillae with developing teeth also occur. The 
pharyngeal teeth in the post-larval goby, -^ inch in length, and in Labrus, T 7 ¥ inch long, 
show interesting stages of growth. In the former species, on both the floor and roof of 
the pharynx, the dental sacs are crowded together ; but in Labrus they extend along the 
under surface of the basi-branchial plate, as well as on the pharyngeal floor. Each tooth- 
sac is formed of deep columnar cells, in which the conical tooth is seated on an internal 
cellular pulp or papilla. The tooth is formed as a hard hyaline cap — semilunar in 
transverse section — which becomes more conical, and finally, when very acuminate, pierces 
both the sac and the epithelium of the pharynx. At the tip the tooth is solid, and a 
little below a neck or ring of clear deeply-stained tissue exists, the hollow portion or 
root of the tooth beginning at that point. Some of the teeth appear to be compound, 
and give off one or more denticles from a common papilla enclosed in hyaline tissue ; 
while in other cases separate teeth occur, in such close proximity that three or more may 
be present in a single fold of the pharyngeal epithelium. The teeth may lie deep in the 
pharyngeal wall; indeed, some of these structures in a young post-larval Labrus are 
separated from the pericardial chamber underneath merely by the thin pericardial layer, 
and in this species the subepithelial tissue of the pharynx is in some parts little else 
than a mass of tooth-sacs. In the post-larval gurnard, ^ inch in length, the cartilaginous 
mandible is completely surrounded by hyaline tissue, and in some of the microscopic 


preparations it appears that this tissue is formed in the perichondrial cellular coat ; indeed, 
near the symphysis the very small deeply stained cells clearly intervene between the 
hyaline deposit and the cartilage itself. The hyaline matter, moreover, does not in 
transverse section exhibit the same cylindrical form as the mandibular bar ; for, though 
rounded on the outer side, it rises into a blade-like ridge on the inner side, while above 
its surface is flattened. 

The maxilla in Salmo presents a different appearance from most of the other cartilages, 
since the cells are indistinct, the markings resembling irregular waves, except at the 
anterior and posterior ends. Between the sixth and seventh weeks the dentary region of 
the mandible shows many teeth, arranged in several rows, the anterior being curved, the 
posterior more or less straight. About thirty teeth occur on the premaxillae and maxillae. 
The mucous membrane is raised into papilliform elevations, the summit of each bearing 
a tooth. The premaxillary teeth are the largest and strongest. The otoliths are at 
this stage the only hard and dense structures in the cephalic region, and they are some- 
what spherical, and present the usual radial striations. The posterior otolith in each 
auditory sac is irregular, and shows longitudinal markings and interrupted streaks. The 
cells of the cranial cartilages are, on the whole, more regular and distinct than those of 
other parts. 

Hyoid Arch. — The embryos of the wolf-fish which issued on 17th January presented 
a well-developed hyoid arch. The glosso-hyal region has anteriorly a large cushion of 
mucous tissue. Three cartilaginous elements form a rounded arch within the latter 
(PI. XXV. fig. 1), viz., a median (glosso-hyal) and two lateral (hypo-hyal). The cerato- 
hyals abut on the hypo-hyals. In all these the cartilage-cells are smaller than in the salmon. 
The cerato-hyals are considerably less than in the salmon, and, indeed, the same may be 
said of the cartilaginous elements in general, and especially of the glosso-hyal. Four 
branchial arches succeed the former, and bear simple though highly glandular papillae 
(PI. XXV. fig. 2). The fifth arch is visible in a specimen on 20th April, as a bar on 
each side in front of the pericardial chamber, and teeth are readily distinguishable on 
1st May. The double rows of branchial pinnae present deep crenations along the sides in 
April, and these form long papillae towards the end of the latter month and in May, so that 
each process is feather-like. Hyaline ossific tissue now appears as a superficial coating 
upon the surface of the branchial cartilages. On 21st May the cerato-hyal and other 
cartilages of this region are bordered with a firm layer of the same tissue. The brittle- 
ness of the branchial arches externally shows that this development has made consider- 
able progress, and the ossific investment is still more marked a month later (20th June), 
when it was difficult to make good sections, on account of the brittle character of the 
skeletal bars, or rather of the hyaline ossific tissue clothing them. The branchial 
papillae at this latter date are richly pinnate. 

The hyoid bar may, as in the gurnard, T \ inch in length, rapidly lose the form of 
a simple rod, and while the mandible is rudely elliptical, the hyoid becomes bilobate in 
transverse section, from the presence of deep grooves, and shows two prominent outer 


ridges and one superior internal ridge. The operculum in the various post-larval forms 
presents no noteworthy points, except in the wrasse, ^ inch in length, in which a dense 
plate of hyaline tissue appears in the tegumentary flap. 

Vertebral Column. — The first of the permanent skeletal structures in connection with 
the notochord in Anarrhichas, are a pair of cartilaginous bars, which spring from the 
upper side of the perichordal sheath at the sides of the spinal cord. These are developed 
in a streaked, cellular region (mesoblastic) very evident beneath the muscular masses of 
the parts. Each bar springs by a broad base from the notochordal sheath, passing 
upwards to form an investment round the spinal cord. Early in February, a more 
regular arrangement of cells takes place at the base, which is triangular in transverse 
section, and in the investment, passing upward laterally, a transverse disposition of cells 
occurs, forming indeed a cartilaginous bar — the neural arch. The firmness of this buttress 
on each side dorsally, is probably the reason why the unprotected median arch of the 
notochord bends upward in the preparation ; and the precursory thickening at the sides 
inferiorly produces the same result in the ventral arch. At this time there are no 
distinct interspinous cartilages, though there are signs of them dorsally. These structures, 
however, appear in March, and towards the end of that month the neural arches meet 
superiorly over a small canal, which has for some time been visible in the region. 
Posteriorly they also send up dorsal processes — the neural spines. Above the latter is the 
interspinous cartilage ; but the two do not touch.*" In the beginning of April, a some- 
what conical cartilaginous process appears at a corresponding region in the lower arch, 
but proceeds only a short distance downward above the segmental duct, being larger, 
indeed, in the posterior than in the anterior region of the body. It gradually lengthens 
so as to pass downward and outward over the subnotochordal region in which the artery 
and segmental ducts lie. By the end of April, or early in May, these processes are 
strengthened by a coating of hyaline ossific matter, continuous with that which has 
invaded the notochordal sheath (PL XXVI. fig. 3, ncs). Instead of collapsing or becoming 
wrinkled in sections, the sheath forms a firm, nearly circular ring. This hyaline and struc- 
tureless ossific matter, like that seen in the premaxillary and clavicular elements, surrounds 
the conical process of cartilage, so that in several sections it forms an isosceles triangle, 
with a long apical process. The two dorsal arches are similarly coated with this matter. 

In post-larval stages of other Teleosteans, considerable variation in the character of 
the elements of the early vertebral arch exists. In Cyclopterus, for instance, the basal 
stumps of the neural arch arise as cartilaginous processes, developed as in Anarrhichas 
within the perichordal sheath, and resting upon the chorda! investment. In the young 
cod, § of an inch in length, the neural arch, as in the wolf-fish, appears to spring as a 
hyaline outgrowth from the tubular investment of the notochord, which is formed of the 
same hard tissue. In Cottus scorpius, when only •§ inch long, the notochord exhibits this 

* These probably correspond with Balfour and Parker's "two bars intervertebral^ placed, — two osseous plates on 
the outer side of these, and continuous with the lateral osseous bars of the neural arch. The former give rise to cartila- 
ginous elements above the osseous bridge of the neural arch in the adult." 

VOL. XXXV. PART III. (NO. 19). 6 Z 


hyaline coat, having the form of a translucent ring, in transverse section, separated by an 
interval from the inner limiting cells of the chorda itself. The neural arch is imperfect at 
this stage ; but the haemal arch, formed of the same hyaline matter, is complete in the anal 
and caudal regions. It is remarkable, that in the tail, the elements of the neural arch are 
formed of cartilage, upon which hyaline matter appears as a thin shell ; but anteriorly the 
arch is formed by hyaline outgrowths solely. A similar condition is exhibited by Cottus 
quadricomis, when -£% inch in length, the cartilaginous outgrowths destined to unite as 
the neural arch in the caudal trunk, having at first a plate of hyaline matter deposited on 
the outer surface only. In the post-larval Labrus, T V inch long, while the anterior part 
of the spinal cord is protected incompletely by the developing neural arch, consisting of 
hyaline basal stumps, and an independently formed hyaline neural spine, bifurcate below, 
the posterior portion of the cord in the tail is still enclosed merely in the membranous 
chordal sheath, destitute of any more permanent element than the rudimentary neural 
spine, which consists of two approximated plates of hyaline matter, clothing a strand of 
connective-tissue. This strand forms dorsally a knob, which is deeply stained in the 
microscopic preparation, and continues to the base of the dorsal fin (in this species 
characteristically lengthened), where a second pair of hyaline plates are developed, viz., 
the rudiments of the terminal fin-rays. The haemal arch below is complete, and encloses 
what appears to be a mass of cartilage, so that it is really a solid ventral process, below 
which pass the caudal artery and vein. In the caudal fin the same features are seen, but 
the connective- tissue strand which passes down from the haemal arch is clothed by hyaline 
matter, two lengthy plates of which form a large hypural, and upon their outer surfaces 
a diagonally directed muscular band is inserted on each side. Each muscle is attached to 
the corium about the level of the haemal arch, and passes obliquely downward. 

The cellular external region of the notochord in Anarrhichas is rendered conspicuous 
by the appearance of the definite hyaline ring above mentioned. A similar ossification 
proceeds in the dorsal fin-rays, in fact, they appear to commence as hyaline bars ; but 
the interspinous elements seem not to do so. The neural spine is likewise ossified. All 
the structures mentioned have become more ossified at the beginning of May, the haemal 
arches meeting posteriorly to enclose the artery and support the interspinous elements 
of the median ventral fin with its fin-rays. The haemal arches in the region near the 
caudal become elongated, the posterior pair being also greatly flattened, for the support 
of the caudal rays. # At a still later stage, towards the end of May, the haemal spines 
have frequently between them a transverse row of cartilage-cells, apparently binding them 
together towards the tip, the evidence of a series of sections showing that amalgamation 
has taken place. The ossified sheath of the notochord is now brittle, and frequently gives 
way under the knife of the microtome. At this stage no special lamellae leave the 
various ossified structures connected with the vertebral column, as Grassi t shows in 

* The firm hyaline ossification here mentioned is by the process called ectostosis. When surrounding the chorda 
in the wolf-fish, it recalls the unsegrnented cartilaginous tube Balfour found round the notochord of Elasmobranchs. 

t "Lo suiluppo della colonna vertebrale ne pesci ossei" {Mem. del dott. Battiste Grassi ; Atti d. R. Accad. dei 
Lincei, seri. 3, vol. xv. pp. 311-337, Taf. i.-viii.). 


the salmon, and it is clear that in all in these parts the ossification is external to the 
cartilage, to which it forms a coating. The dorsal and neural arches present numerous 
cartilage-cells inside the osseous investment, where they spring from the column, but a 
transverse row of large cartilage-cells occurs beyond this, and the whole at the end of 
May is enclosed by a dense hyaline ossific layer. This hyaline sheath can often be cut 
without fracture, so that it contains certain plastic elements. 

At the latest stage examined (as shown in specimens that survived till nearly the end 
of June), after maceration, the vertebral column is found to consist of rings of firm 
hyaline osseous tissue of considerable thickness, with spaces between the serial vertebrae. 
To the dorsal and ventral aspects are attached the neural and haemal arches, which meet 
and give rise to the spine. At the junction of these arches is a groove marked with 
transverse striae — an aspect probably due to the remains, still visible, of the cartilaginous 
elements. A similar translucent region occurs at the base of each pillar of the arch, at 
the vertebral ring, and probably arises from the centre, now wholly removed. A com- 
parison of this with the vertebral column of the young cod, obtained in tidal creeks in 
the beginning of June, will show how advanced the latter species is. The vertebrae of 
the cod are ossified from end to end externally, even more so than is indicated in Grassi's 
figure of the bleak (op. cit., pi. vii. fig. 8), so as to be separated only by a thin inter- 
vertebral edge, and the notochord is reduced internally to a moniliform band. 

In the newly-hatched salmon, cartilaginous spines, neura- and hsemapophyses, occur 
on each side of the notochord above and beneath. They are fixed to the sheath of the 
notochord, penetrate one-third its depth, and are best marked anteriorly. About the 
sixth week the vertebral column presents a moniliform appearance, the neura- and 
haemapophyses being adherent, but not continuous. Under pressure a tough moniliform 
cellular central portion escapes, — the remnant of the notochord. The column has now a 
distinctly barred appearance from the development of the vertebrae, each vertebral bar, 
moreover, bisects the nail-shaped head of the neura- and haemapophyses. These last 
named elements are exactly opposite each other in the darker portions, and on reaching 
the column the terminal process expands into a nail-like head, having on each side a 
pointed tip, devoid of cartilage-cells. The processes retain nearly the same shape as 
formerly described at a much earlier period. At the present stage, however, the cells are 
smaller and more crowded, and their nuclei are more distinct. At the tips of the larger 
ones, near the tail, the cells are smaller, and more flattened, as well as more densely 
arranged. Grassi* enumerates in the salmon and others four layers from within out- 
ward, — (1) cellular stratum of the notochord, (2) epithelial coat, (3) proper membrane of 
cord, (4) fine elastic amorphous membrane, — which is developed from the surrounding 
embryonic connective-tissue. 

Ribs. — Little can be said in the way of addition to information already existing as 
to the development of the ribs. They form long bars of finely cellular cartilage, at the 
stage of date — 20th June, when the ring of hyaline ossific matter surrounds the notochord, 

* Op. cit. 


and, as Balfour and Parker observe, they are modifications of haemal processes, as 
indeed had previously been noted, amongst others, by Muller and Gegenbaur. 

In the salmon, at the sixth or seventh week, cartilaginous ribs are present, and show a 
well-formed head articulated to the parapophyses by a broad surface, apparently having 
some elevations on its otherwise straight edge. The attached end is widened and shows 
numerous cells ; but distally a single row of cells gives the tip a scalariform appearance. 
In a few instances, the cellular structure was disconnected in the centre of the rib, the 
intervening band consisting solely of a cord of the transparent matrix. The same has 
been noticed by Grassi in a Cyprinoid. 

Brain. — In Anarrhichas, as in most Teleosteans, the chief features in the brain are the 
great size of the mesencephalon, and the depth of the entire brain-mass. Probably con- 
nected with this is the much more marked flexure of the anterior end of the notochord (PL 
XXIV. fig. 1) in the larval wolf-fish than in the salmon. The fore-brain is shorter in its 
antero-posterior diameter, and presents a comparatively larger area (in transverse section) 
than in the salmon. At the origin of the optic nerves the proportional area of the brain- 
surface is larger, and the breadth of the roof (mid-brain) greater than in the salmon. 

The shape of the brain-mass shows, indeed, great variation in post-larval stages of 
various species. In the cod, T % inch long, it is, like Anarrhichas, rounded and com- 
pact as a whole — the optic lobes in cross-section forming a semicircular mass — almost 
equally composed of an upper layer of white matter, and a lower layer of deeply-stained 
vesicular matter, the line of separation between them passing parallel to the surface of the 
lobes. Posteriorly the roof and floor of each optic lobe thins out very much. In the 
post-larval stage of the gurnard, about T % inch in length, the form of the brain is very 
different — the conformation of the cranium being much flatter than in the Gadoid — the 
brain-mass, especially the optic lobes, are markedly depressed and of disproportionate 
superficial extent, and much thickened in the lateral portion. The vesicular and white 
unstained parts form two well-marked strata of about equal thickness, a condition just 
noted in the optic lobes of the Gadoids. As might be anticipated from the form of the 
adult head, the post-larval Callionymus, -^ inch long, shows a brain considerably de- 
pressed, especially in the middle line; but in the herring, when of slightly less size (viz., 
-£-% inch), the flattened condition is even more remarkable, the optic lobes spreading out 
laterally, and having thus a large superficial area. Hardly less striking is the condition 
of the optic lobes in a form, probably Ammodytes (^ inch long). They are more rounded, 
but, on account of their disproportionately large size, present a superficial surface quite as 
noteworthy as the preceding forms. The deeply-stained vesicular matter forms a much 
thicker stratum than the unstained white layer, and the line of separation is somewhat 
irregular, and does not pass parallel to the superficies of this region of the brain. These 
two layers in the optic lobes of the goby, ^ 5 ¥ inch long, differ still more from the Gadoid 
and other examples mentioned above, the vesicular stratum in transverse section pass- 
ing upward to a median point, and forming a deeply angular mass of great thickness, 
sharply marked off by the sloping line of separation on each side from the thin and 


dorsally-rounded whit estratum above. The apex of the grey substance in this early 
post-larval stage quite separates the white matter into two masses, and this condition 
is still more marked in a later post-larval stage, the vesicular matter intruding to a 
larger extent in the goby ^ of an inch in length. 

The superior fold of the mid-brain (optic lobes) in Anarrhichas forms from side to 
side a semicircle, and is therefore larger than in the salmon. Moreover, the prolongations 
of this fold on each side towards the cerebellum are longer than in the salmon. The 
adjoining edges of the optic lobes (forming the sulcus longitudinalis superior) are some- 
what regularly and deeply crenated, as if indicating rudimentary convolutions. The 
posterior mesial fold (valvula) of the same region is likewise larger, and it gradually 
widens out posteriorly until it merges in the furrow of the medulla. 

Anteriorly the brain presents two somewhat short cerebral lobes, with a large median 
ventricle, which terminates a little behind the anterior border. In vertical transverse 
section they are seen in PI. XXIV. fig. 5. 

The lower margin of the cavity of the fore- brain is closed by a thin cellular layer in 
the middle line in front of the optic commissure. A well-marked commissure passes 
between the two lobes a little above the inferior border posteriorly, and the fissure in 
transverse section is thus closed up inferiorly in the region (PI. XXIV. fig. 5, ac). In the 
section, however, only a portion of the commissure is visible. This may be, as Balfour 
and Parker suggest in Lepidosteus, the homologue of the anterior commissure. The 
olfactory nerves (PI. XXIV. fig. 4, i) spring from the anterior end of the cerebral lobes, 
and their separation is well shown in the same figure. 

Very soon afterwards the optic fibres from the lower part of the brain cross (without 
decussation) in the middle line. 

At the end of March the optic nerves are hollow in some, a considerable chamber 
occurring in the centre from the choroidal fissure to the optic commissure. It must be 
stated, however, that in earlier stages this was not always visible, probably because the 
degree of development varied so much. 

While in many post-larval forms, e.g., the goby, when ^ inch in length, the optic 
nerves are solid ; in the gurnard, ^ inch in length, they exhibit a well-marked longi- 
tudinal fissure shortly before penetrating the sclerotic layer to enter the optic chamber. 
This chamber is even more distinct and capacious in Labrus, T 7 F inch in length, and at the 
point where the nerves pass over each other the lumen is of an irregular form, as its 
walls are much wrinkled, the hollow nerve showing a series of folds, which disappear as 
it passes outward, the walls approaching so as to enclose merely a narrow median slit, and 
this can be traced only to the aperture through which the nerve enters the eye. In 
Callionymus, ^ inch in length, the optic nerves are surrounded by a layer of fibrous 
tissue having a slightly metallic lustre. 

Some further points of difference present themselves in regard to the olfactory nerves 
in certain stages. Thus, after they have separated from thef ore-brain in the manner 
described in Anarrhichas, they may pass downward to penetrate the lateral wings of 


the trabecule, as in the post-larval gurnard, ^ inch in length, in which species the 
fronto-nasal cartilage is very dense and massive. Through this cartilage the nerve 
goes by a distinct canal — a similar canal, it may be noted, passing along a parallel 
course slightly external to the nerve, and giving transit to an artery. In the cod the 
nerves pass along the floor of the cranium, and, without piercing the anterior trabecular 
outgrowth, reach the olfactory pits. The nasal pits show considerable variation in their 
rate of development, e.g., in a Clupeoid barely \ inch long, in which a cartilaginous cup 
is already fairly formed beneath the olfactory organ; but in the goby, about \ inch 
long (^ inch), the walls of the chamber, though very thick and composed of elongated 
radially-arranged sensory cells, have merely a very thin outer membranous support similar 
to the delicate layer lining the pit. In the outer portion of the wall of the organ large 
loosely connected cells appear, forming a distinct prominence, probably indicating the 
transverse bridge, the later development of which in Anarrhichas is described on p. 918. 
The cartilaginous optic cup is well developed in the early post-larval stage of Pleuronectes 
fiesus, that is when the young fish measures ^ inch in length. 

Behind the optic commissure in the wolf-fish a strong band of fibres passes from side 
to side along the ventral edge of the brain (PL XXIV. fig. 6, fa), forming a broad bridge 
of communication. It disappears about the commencement of the succeeding region. 
The roof of the anterior cerebrum is composed at first of a layer of nerve-cells, which 
becomes thinner as the chamber above the inferior pale median streak becomes larger. 
So thin is it in a line with the anterior commissure (PI. XXIV. fig. 5) that, if it is no 
better developed in the Ganoids, Wilder would very readily suppose it to be absent. 
There can be no doubt, however, of its presence in the Teleostean embryo, for the spindle- 
shaped cells can be followed upward to each edge, as a diminishing column that runs into 
the thin layer of more flattened cells forming the roof. It is certainly remarkably thin in 
some parts. It rapidly increases in thickness as the thalamencephalon is approached (PI. 
XXIV. fig. 6, and also in PI. XXIII. figs. 3 and 3a). As in the Ganoids, a transverse 
commissure appears in front of the pineal gland on the roof of the vesicle formed by the 
anterior portion of the thalamencephalon. Posterior to the gland are other bands of 
fibres (PI. XXIII. fig. 3) crossing over the arch beneath the optic ventricles, and above 
the continuation of the third ventricle. Behind the foregoing is the large posterior 
commissure, which in front commences over the infundibular region, and it increases in 
size when traced backward. 

The optic lobes posteriorly form a conspicuous vesicular region on each side, from the 
great size of the optic ventricles, and on the floor, in its progress to the median fold, are 
the tori semicirculares (PI. XXIV. fig. 3), fusiform thickenings very characteristic of the 
region over the commencement of the notochord. The arrangement of these folds in 
Anarrhichas is noteworthy, and differs from the same folds in the salmon whether one, 
thirteen, or forty-five days old, but the nature of the preparations may account partly for 
the divergence. The latter species (salmon) shows a nearly straight section of the floor of 
each optic lobe posteriorly, and the fusiform enlargements and distinctness of the median 


dorsal folds (valvula cerebelli, fornix of Gottsche, which seem to be present only towards 
the posterior border) are all much more evident in Anarrhichas. Behind this, the 
inner or inferior surface of the outer layer becomes richly folded, while the fusiform 
condition of the lower fold disappears, a nearly straight band taking its place. These 
folds continue until the optic ventricles disappear. In the salmon of the first day there 
are only a few smooth folds on the under surface of the roof of these lobes. 

In the region of the pituitary body in Anarrhichas, the trabeculse are more elongated 
and more obliquely situated than in the salmon, where they are also more widely separated. 
The pituitary body is perhaps less flattened from above downward than in the salmon, 
but in both forms it has the same structure, viz., aggregated nucleated cells (PL XXIV. 
figs. 1 and 2). A band, apparently of connective-tissue, proceeds from the under surface 
of the brain to the pituitary body, and this band is in the main fibrous ; while in the 
salmon it is composed of connective-tissue cells, the nuclei of which stain very deeply. 
A layer of connective-tissue separates it from the infundibulum. 

In each of these forms the pineal gland — both as to position and histological 
structure — is the same, but the pale commissural fibres passing to the upper region of 
the mid-brain are proportionally larger in Anarrhichas, and therefore can be traced 
further into its substance (PL XXIV. fig. 1, pf). The central aperture of the thalamen- 
cephalon in front of the pineal gland is more capacious and better defined than in the 
young salmon, and it rapidly assumes large dimensions, and opens into the third ventricle 
inferiorly (PL XXIII. fig. 5, and PL XXIV. fig. 2). The ventricles of the optic lobes 
appear superiorly at the side, as in Anarrhichas, and the two rapidly merge into one 
chamber. The separation of the infundibulum from the peduncular region is more 
distinct, the connective-tissue being very apparent; but the arrangement of the parts is 
similar to that in the wolf-fish, though the infundibular cells are larger. 

The complexity of the brain, especially in regard to minute structure, augments as the 
larva increases with age. Thus, in the middle of March the radiate fibres of the mid- 
brain (optic thalami) to the optic lobes are largely developed, and the thicker cortical 
region is more clearly differentiated from the inner layers. Moreover, it is evident 
that the tori semicirculares posteriorly are now encroached upon by commissural fibres 
passing to the roof. 

A distinct and broad commissure appears in front of the pineal body, and is continued 
behind it, the posterior band being very evident, so that it may even be regarded as 
separate. Horizontal sections do not, however, sufficiently aid in deciding this point. 
Next, from before backward, is the commissure of the optic nerves inferiorly and in the 
neighbourhood of the infundibulum. Lastly, are other transverse bands which will be 
subsequently mentioned. 

Somewhat later a differentiation appears in the centre of the peduncular region, in 
the form of a rounded area with a cellular margin (PL XXIII. fig. 6); the vesicle in 
front and beneath the pineal gland becomes more or less obliterated; and the optic 
ventricles posteriorly show a diminished lumen on each side, for the fibres passing to the 


outer border of the fusiform tori semicirculares, and in part through them, considerably 
encroach on the cavity and bind the roof more closely down. On 20th June a more 
distinct differentiation of the anterior ends of the fore-brain into anterior cerebral and 
olfactory lobes (PI. XXIV. fig. 4) is noted, and the area behind is more complex, cells 
being developed chiefly on the upper and inner edges, that is, on the sides of the median 
ventricle as seen in transverse sections. The median arch or roof of the fore-brain is 
considerably narrowed in front by the great increase in the nerve-mass at each side, so 
that a mere chink remains. On each side of the central fissure superiorly the margin 
of cylindrical cells trends upwards, and ends in a point, then bending outwards, 
leaves an acuminate projection in transverse section. Externally the arch is com- 
pleted by a layer of tissue containing pigment — continuous with the " pia mater," and 
internally by another layer, similar in structure, and therefore differing in appearance 
from nerve-tissue proper. This indifferent tissue is probably the anterior boundary of a 
chamber or vesicle which immediately appears in the median line in front of the pineal 
gland, and is possibly homologous with that described by Balfour and Parker # in the 
roof of the thalamencephalon in Lepidosteus. The vesicle is preceded by a double papilli- 
form process of the roof, which apparently soon coalesces inferiorly to form the vesicle. 
The entire process is formed of nerve-cells, and when the vesicle is fully developed, these 
present in transverse section a somewhat columnar arrangement, and the wall is sym- 
metrically folded superiorly. The band of nerve-tissue forms the floor of the vesicle or 
chamber just mentioned and the roof of the ventricle, for the anterior lobes present a 
peculiar layer of large nucleated cells under the columnar series lining the thalamencephalic 
chamber. This is especially noticeable in the line of the section shown (under a low power) 
in PI. XXIII. fig. 3a. The walls of the vesicle are quite distinct from the optic thalami 
at the sides, but gradually (as we proceed backward) they merge laterally into the optic 
thalami, then the roof becomes a mere bridge between them, each end being thoroughly 
incorporated with their tissue; while the floor, assuming a doubly fusiform shape (i.e., 
thinner in the middle and at each end), soon disappears. In the drawing (PI. XXIII. 
fig. 3) the separated ends of this floor are seen — the knife having probably caused 
rupture. When it (the vesicle) first appears in section comparatively little brain tissue 
lies externally and very little inferiorly, while superiorly only the thin roof of the 
vesicle occurs under the pia mater. Proceeding backward the nerve-substance (optic 
thalami) at the sides becomes massive, and the vesicular space enlarges in the median 
line. Moreover, a papilliform process (pineal gland) makes its appearance in the centre 
over the roof of the chamber. A succeeding section (PL XXIII. fig. 5, possibly 
somewhat oblique) is instructive, showing the tapering points of the mid-brain touching in 
the middle line, the cellular pineal body appearing immediately beneath, while along the 
roof of the thalamencephalic chamber a commissure connects the central pale fibrous 
region of the commencing optic thalami on each side. An unusual development of 
connective tissue (neuroglia) could only be confounded with this connecting band, and 

* Phil. Trans., 1882, pt. ii. p. 376. 


there seems no reason to alter the view just mentioned.* Moreover, this commissural 
band of fibres is observed in several succeeding sections as it spreads over the roof of the 
ventricle between the optic thalami, the sides of the chamber being now solid masses 
of nerve-tissue (optic thalami). This section further indicates the opening of the 
thalamencephalic vesicle or chamber into the common (or third) ventricle, extending to 
the anterior lobes in front and the optic thalami in the region under consideration. 
In the next section a cellular baud proceeds in the median line of the roof towards the 
margin of the arch, apparently the remnant of the pineal gland formerly mentioned, a 
few of the commissural fibres being still visible beneath. Just behind, the median parts 
of the lobes of the mid-brain become broader, and are separated inferiorly from the central 
process ; moreover, fibres pass between them and through the median process, forming a 
distinct commissure. A space lies on each side of the latter inferior] y, the roof being formed 
by the lobes, the floor by a more or less fibrous band on each side connected with the 
median process between the lobes (PL XXIII. fig. 5). An indication of a space, possibly 
due to the mode of preparation, occurs below the latter band, viz., at the point marked 
sp. on PL XXIII. fig. 5, and then the cells forming the lining of the great central ventricle 
in the thalami occur. As the commissural fibres last spoken of diminish, the central 
body becomes more clearly differentiated from the edges of the lobes of the mid-brain, so 
that it is somewhat awl-shaped, narrower above, where it joins the lobes in the middle 
line, dilating in the free middle region, and again narrowing at its attachment to the 
roof of the inferior region of the mid-brain. In structure this median region is cellular. 
In the next section it is bell-shaped, the handle of the bell being superior. The cells are 
also symmetrically arranged, viz., a thick layer of large cells externally along the margin 
of the bell, and a broad median band of large cells. The latter arrangement, which is 
broadest inferiorly, may indicate a double process [i.e., the coalescence of a structure 
originally double). The sides of the structure become continuous inferiorly with the floor 
of the mid-brain (optic thalami), and its base rests on the commissure (with the probably 
artificial aperture), for separation of the two strands might easily occur in the lax tissue 
(neuroglia). This separation of the commissure into two layers is a marked feature, and 
cells occur between them as they debouch into the thalami (PL XXIII. fig. 5). 
Posteriorly the median process becomes more cylindrical, narrows inferiorly, then hangs 
like a leaf-shaped structure from its stalk between the optic lobes, its double nature being 
shown by its pale central region on each side and the two rows of median cells. It 
diminishes to a mere papilla and then disappears. 

The commissural fibres between the optic thalami (in the larva of 20th June) appear in 
transverse section along with the pineal gland; they are thus in the same plane, and not in 
front of it, as in the Elasmobranchs, and at first they have a median cellular mass, which 
is torn in the preparation studied — causing the cavity before mentioned. The pineal 
gland lies above the fibres, and below them is the fissure of the mid-brain continuous with 
the Aqueductus Sylvii. The fibres are thickest towards the posterior part of the gland. 

* What connection this commissural band may afterwards have with the pineal gland is uncertain. 
VOL. XXXV. PART III. (NO. 19). 7 A 


The commissure on the roof of the lower division of the mid-brain (thalami) forms a 
uniform deep band after this, and then it ceases, so as to open up the whole central region 
into a single chamber (common ventricle). 

In longitudinal sections the roof of the tbalamencephalon shows a large fold of cells 
(tela choroidea) in front of the pineal gland. The latter is cellular (on 1st May), and is 
connected by strands of fibres at its base with the centre of the optic thalamus, the 
direction of the fibres being generally downward and backward. 

At this stage the posterior region of the fore-brain is protected externally by a special 
plate of cartilage, which stretches over and extends for some distance down each side, 
passing over the anterior end of the optic lobes. The black pigment lies beneath it, while 
externally is a hyaline ossific stratum. In transverse sections a deep dimple is usually 
present over the pineal gland. The next feature of note is the formation of the optic 
commissure. This is produced by a great band of fibres arising at the upper border of 
the thalamus, and apparently continuous with the optic lobes above. This massive band 
passes down on each side of the central region of the thalamus and the fibres simply 
cross each other beneath. A streak in the centre of the band inferiorly is probably the 
lumen of the nerve-trunk, and it can be traced to the choroidal fissure. The succeeding 
sections show oblique bundles of longitudinal fibres on each side of the central region 
inferiorly. These are isolated by the vertical bands, some of which form a commissure 
beneath the central chamber of the thalamus. In the line of the great transverse com- 
missure, just behind the pineal gland and at the upper part of the thalamus, a circular 
region appears in the centre of the lateral mass of the organ superiorly, which increases 
in size as it proceeds backward. Its outer (cellular) region stains deeply, and in the 
preparation it has separated from the surrounding tissue. Vertical bands of fibres pass 
across each region in front of the pituitary body. Shortly after the latter appears the)' 
diminish in size, and, keeping at the upper part of the infundibulum, disappear before 
the pituitary slices do. By the differentiation posteriorly of the upper region into 
crura cerebri, these rounded bodies are placed in transverse section at the upper part of 
the infundibulum. The lateral fissures from the infundibulum appear before the 
former cellular differentiations cease, and they by and by attain a larger size than the 
central one, apparently from constriction of the latter. This arrangement indicates 
a possible homology with the trifid infundibulum of the Elasmobranchs, and the so- 
called lobi inferiores are thus apparently closely connected with the infundibular 
apparatus. The central fissure of the infundibulum has a very definite contour, and, 
inferiorly, it leads to the pituitary body, above which a lozenge-shaped dilatation occurs, 
the margin of the aperture being directly continuous with the hypophysis, the centre 
of which is dimpled superiorly by the tip of the lozenge-shaped space. By and by 
(proceeding backward) this central infundibular aperture becomes trifid by the protru- 
sion of lateral diverticula, each forming a separate pouch after the connection with the 
pituitary body is broken. These diverticula soon disappear and the central aperture 
becomes very small, and at the same time its walls are folded and defined by cylindrical 


cells. The folds then increase in complexity, so that in transverse section the 
contour much resembles the uneven outline of the mucous coat of an alimentary organ 
(PI. XXIII. fig. 4), though the folds are less elaborate than those shown by Stieda 
in the turbot. # Meanwhile the two lateral apertures in the lobi inferiores have 
descended, so that the inner or infero-median wall touches the folded central chamber. 
Their central area contains cells in a streaked matrix of protoplasm or coagulable sub- 
stance. The folded mid-region is somewhat triangular in outline, and continues to 
increase in size, while the lateral apertures gradually diminish, for the cells soon cover 
the entire area. After the disappearance of the lateral apertures, the infundibulum 
becomes more elongated transversely, so as to resemble ultimately a transverse bar, and 
then it disappears. 

About the region of the splitting of the infundibular tissue, and before the crura 
separate therefrom, the cerebellum appears under the median folds of the optic lobes. 
It has dorsally the aforesaid lobes, and ventrally the fourth ventricle. 

The organ is marked dorsally and ventrally by a median depression, so that from 
the first it is bi-lobed, and this condition is soon better marked by the occurrence of a 
median fissure. It diminishes and disappears posteriorly above the point where the 
pharyngeal teeth occur in the section. 

When the tip of the cerebellum (valvula) first appears in transverse section below 
the tecti lobi opticij the massive area beneath has the two cellular differentiations just 
below the middle, and the trifid fissure of the infundibulum inferiorly. An intimate 
decussation of fibres takes place in the lower half, transversely as well as vertically 
(PI. XXIII. fig. 6), in the larval fish of 20th June, while the upper half is split by a 
median furrow, the edges of which, however, are closely attached. Then signs of 
separation occur between the upper and the lower divisions, a mass of strong transverse 
fibres passing below the former, and curving upwards at the sides externally. They close 
the median furrow superiorly, having beneath them in the same region a mass of grey 
matter. These commissural fibres increase in bulk as we proceed posteriorly, only a 
median notch existing in the floor above. This continues backward to the medullary 

Shortly after the anterior fold (valvula) of the cerebellum has become distinctly 
double, a strong band of transverse fibres passes over the roof of the ventricle to the 
lateral regions. They proceed on each side to the region of the tori, spreading out into 
the grey matter. After an interval, in which a change in the vault has occurred, for 
the tecti lobi optici have now become lateral in the sections, while the median line is 
occupied by the cerebellum and valvula, another strong band of fibres passes across the 
same region, the direction in the lateral region being chiefly downwards. 

In the cerebellum behind this, various curved fibres cross and vertical bands course 
from above downward. Towards the termination of the organ a median fissure occurs. 
Moreover, a bridge of nerve-tissue is thrown over the floor of the ventricle from side to 

* Zeitsch. f. w. Zool, Bd. xviii. p. 44, Taf. ii. f. 30. 


side, and probably represents the commissure between the lobi posteriores, though these 
are not conspicuous. 

The median fissure of the medulla is at first dorsal and very large, but as the organ 
diminishes it assumes a lower as well as more central position, and becomes much 
smaller. *^ 

Spinal Cord and Lateral Line. — In the extreme caudal region the posterior (dorsal) 
fissure of the cord, especially well seen in the wrasse, T 7 ^- inch long, is reduced to a mere 
central canal, circular in transverse section, and surrounded by vesicular matter ; indeed 
the white matter almost wholly ceases, and the column continues to its termination as a 
ganglionic tube, whose diameter is about one-quarter that of the notochord. This predo- 
minance of the grey matter in the hind part of the spinal cord is a character familiar in 
higher forms. 

About the level of the notochord in the cod, ■§ inch, in length, a canal internal to the 
corium passes along the outer edge of the septum, dividing the two median lateral 
muscular masses in the caudal trunk. It is of small diameter, though very distinctly 
marked on account of the presence of a sheath of black pigment, which continues into the 
intermuscular septa, and indicates the course of the delicate nervous strand connecting the 
canal, no doubt, with the spinal cord. Such connection cannot, however, be clearly made 
out, as the pigment passes only a short distance inward towards the cord. It is to be noted 
that the spinal cord has similar dark pigment in its protective tunic. The preparations 
do not show serial openings to the exterior at this stage, and the lumen of the canal is 
filled with loosely aggregated deeply-stained cells. In Lahrus, T 7 g inch long, a canal 
cannot be made out in the caudal trunk ; but an aggregation of cells occurs beneath the 
integument, on a level with the lower border of the vertebral column. They lie below the 
pigment-layer of the skin, which stains deeply, and show evidence of nervous connection 
with the spinal cord. The cells are large and folded, their walls being pushed in — in the 
form of a figure 8. No lumen in this instance can be discerned. 

Ear. — The general form of the ear of the wolf-fish on hatching is shown on PL XX. 
figs. 2 and 4, and on PL XXL figs. 1 and 4. Cartilage develops much more rapidly in 
the salmon than in the wolf-fish in the otocystic region. Thus, in cutting the ear, 
both anteriorly and posteriorly, several sections show a complete investment of cartilage 
in the salmon ; whereas at a similar stage in the wolf-fish the thin cartilaginous floor goes 
only a short distance upward externally, and at no part is a ring of cartilage completely 
formed. The inner margin of the cartilaginous floor of the ear bends downward 
posteriorly, and continues into the parachordals, which lie on each side of the notochord. 
Shortly behind this it also joins the hyomandibular cartilage, passing towards the middle 
line and disappearing. The general arrangement of the ear is similar in both species, 
though at no period does the inner border of the capsule pass so near the middle line in 
the wolf-fish as in the salmon. This is clearly seen in the neighbourhood of the noto- 
chord. The structure of the nervous cushions and their auditory cilia or stiff protoplasmic 
processes in the anterior and posterior chambers present no feature of note. 


The properties of the fluid of the chambers are quite adequate for the development 
of the otoliths toward the internal wall. It is sufficient to point to the growth of the 
Nemertean stylets to prove how perfectly such organs can be produced in successive 
generations of the species. 

On 20th February a considerable increase in the otocystic cartilage of the wolf-fish 
had taken place. From before backward this is marked superiorly by an increase in the 
cartilage behind and above the eye, and inferiorly by a thickening of the bar behind. 
The great development of the cartilage of the basilar plate around the anterior end of the 
notochord gives this inferior bar a more distinctly horizontal position, one of its chief 
flexures being caused, indeed, by the carotid. 

On 16th March the cartilaginous mass anteriorly and superiorly has increased so much 
that it presents a complete ring in section, and thus somewhat further back the cartila- 
ginous boundary is complete externally, while internally it ends abruptly opposite the 
lower border of the optic lobe. It then sinks downward and presents a large perforated 
median prominence below the ganglionic mass of the fifth, by which the auditory 
nerve passes into the cavity. The latter contains large ganglion-cells and hyaline 
coagulable lymph. After this swelling the cartilage diminishes internally, and ends on 
each side of a median mass of connective tissue below the infundibulum. Beneath its 
termination is a space and then an artery and vein (carotid and jugular). The pointed end 
of the notochord begins above this median band of connective tissue. By and by the car- 
tilaginous floor, now somewhat thin, glides in towards the notochord, and almost coalesces 
superiorly. At this part it forms a thin wall externally, and ends in a thickened region 
about half-way up, and the latter increases from before backward over the posterior semi- 
circular canal, until a complete ring is formed round it, only a slender bar connecting the 
latter inferiorly with the somewhat thickened end at the notochord. The contrast also 
between the massive inner wall of the canal and the thin outer wall is marked. Then 
(proceeding backward) the ring is broken internally, and only a slender line externally 
connects the thickened superior loop with the thin horizontal bar leading to the noto- 
chord. The superior mass diminishes and disappears, and the outer part of the horizontal 
bar becomes disconnected from the denser region at the side of the notochord, the poste- 
rior boundary of the ear being formed by a thickened mass of cartilage joined to the arch 
over the spinal cord, and again forming a ring, as in front, before terminating. 

A month later considerable change had occurred in the otocyst — the chief feature 
being the diminution of the cartilage, which is now covered by a layer of hyaline ossific 
tissue, running from the notochord over the floor, and along the thin layer of cartilage 
externally, but the thickened mass to which the hyomandibular is joined does not show 
it distinctly. As soon as the bar becomes nearly horizontal (sloping a little upward and 
outward from the cord), this hyaline coat extends from the cord to the upper and outer 
border of the ear. The three semicircular canals seem to be similar to those in the previous 
stage, but the ganglionic aperture in the floor anteriorly is less distinct, and the ganglion 
lies more completely in it. The head of the hyomandibular is rounded, and the cartila- 


ginous support of the ear is thinner and more shapely. Membranous lamellae divide the 
ear into the three spaces ; the external chamber is covered by cartilage, and the 
posterior follows in the same manner as before. The large otolith lies at the inner and 
inferior angle near the notochord. 

A month subsequently the most noteworthy alteration is an increase in the amount 
and brittleness of the hyaline coating on the cartilage, and the same may be said for the 
succeeding month (June), fracture of the supporting skeleton of the ear frequently taking 
place in sections. The cartilage has diminished as a whole, its cells have become finer, 
and the brittle hyaline layer has increased in bulk. One of the most brittle regions is the 
inferior wall of the cavity lying to the inner border of the hyomandibular articulation. 

Olfactory Organ. — The chief point examined in connection with this organ was the 
formation of the two nasal apertures. In the earlier stages the single nasal slit assumed 
a vertical position, and at the beginning of April was of large dimensions. About the 
6th of the latter month a slight promontory was noticed in the middle of each lip of the 
fissure, and in ten days the promontories had met so as to make an aperture on each side. 
Each aperture on the 1st May was surrounded by an elevated rim, and the bridge had 
now become broad. At first the nasal slits lie in a hollow between the eyes, but at the 
latter date the snout projects further forward. The usual irregularities were observed in 
a series of specimens, some having the single slit on 21st May and with a considerable 
yolk-sac, others with the apertures fully formed — as just described. 

In Pleuronectes Jlesus, T % inch in length, the olfactory lobes are somewhat distant from 
the terminal sac, and the olfactory nerves pursue a course, parallel to each other, between 
the posterior process of the rostral cartilage and the trabecular. On the floor of the 
cranium the two nerves rest upon a loose connective meshwork, and further back they 
bend inward, to unite with the olfactory lobes at the point where the superior and inferior 
oblique muscles of the eye have their origin on the cornu trabecular. 

A promontory on each lip of the nasal aperture has been already described in the 
goby, -^ inch long, large loosely aggregated cells forming an outgrowth from the radially 
disposed cells of the olfactory epithelium (see p. 910). In the post-larval wrasse, y 7 ^ 
inch in length, the transverse septum is complete, and the anterior and posterior nares 
are now distinctly separated. 

Sensory Organs in the Snout. — Remarkable sensory organs occur on the snout of 
the embryonic haddock (PI. XXI. fig. 7), and are developed on the maxillary 
and mandibular elements in the post-larval gurnard. When the latter has 
reached the length of -£% of an inch, sections of the maxillary bar show organs like the 
sensory cushions in the otocyst or the papillae along the dorsal and lateral surfaces of the 
trunk. The maxilla in section has the form of a flattened plate of hard hyaline tissue 
placed obliquely. This oblique bar gives off an upper arch, which bends over to meet a 
short crest sent up from the ventral margin of the bar. A rude tube, very angular in 
transverse section, is thus formed, but its outer wall is completed only at intervals. In 
the tube are seated certain sacs, on one side of which a cushion of columnar epithelium 


is placed. Sensory hairs appear to occur on the surface of this cushion. Similar sensory 
organs also occur in a tube hollowed out in the mandibular cartilage. These sensory 
organs communicate with the exterior, since the tube in which each is placed is in its 
posterior part incomplete below, and the columnar epithelium on the roof of the cavity is 
directly exposed to the surrounding medium and to stimuli from the outside. 

Alimentary System. — The mouth in the larval Anarrhichas presents a different 
shape from that of the salmon, and on emergence from the egg it is moreover widely 
open, forming a lozenge-shaped aperture when viewed from the front, and its border is 
rigid and motionless. Its upper angle (the premaxillary elements) is considerably elevated 
dorsally, and is depressed between the very large eyes. In this form and in the salmon a 
peculiar flapping of a process is observed in respiration, and in sections of both species an 
extension of the mucous membrane of the mouth hangs down and projects inward from 
the jaws on each side of the trabecule (PL XXIII. fig. 3, Jim). This membrane forms a 
complete floor in front, but posteriorly it runs into lateral flaps, which become continuous 
with the mandibular articular process. In Anarrhichas the smooth fold of this flap poste- 
riorly, as its free tip bends in to join the upper surface, indicates a definite differentiation. 
In the young salmon the membranous process waves synchronously with the movement 
of the mandible. The anterior margin of the mouth, in transverse section, forms at first 
a well-marked groove with the lumen of a vessel in each mandibular elevation. It is to the 
latter that the soft tissues of the mandible are soon attached. The mucous lining of the 
roof of the mouth, from the point of junction of the mandible, is comparatively thin, 
the cells being chiefly of the tesselated variety. 

In Cottus, § inch long, the oral mucous membrane shows large glandular cells, spher- 
ical in form, and disposed along the numerous rugse which are better developed in 
Callionymus. In this last-named species, when ^ inch in length, the mucous layer is very 
thick and the rugse most marked. A great increase of glandular epithelium may occur in 
the lining of the branchial region, and in Labrus, T 7 ^ inch long, it forms a layer of some 
thickness at the upper angle of the opercular flap. The mucous coat becomes thicker, 
and shows columnar cells in Anarrhichas, when the notochord appears in section, and 
the submucous connective tissue increases in volume, so that the internal lining is readily 
thrown into frills. Outside the sub-mucous coat is a layer of circular muscular fibres. 
The frills of the mucous membrane are rendered more prominent by the grasp of the 
increasing circular coat, and the canal is diminished in calibre, so that it forms a com- 
paratively small tube (PL XXVII. fig. 6). The columnar and finely granular epithelial 
lining also has considerably increased in thickness. 

In the post-larval Gadoid, § inch in length, the general structure of the oesophageal 
wall is similar, but the grooves in the roof of the pharynx are deep — two ridges projecting 
very prominently — while on the floor three or more ridges occur. The epithelial coat 
passes into the grooves, and is very largely developed — the bulky mucous cells resting 
upon a loose meshwork of connective tissue, with apparently some longitudinal muscular 
elements. A little further back glands become more abundant, and an outer tunic of 


circular fibres appears. This portion of the alimentary canal lies in a spacious recess in the 
liver, and two lateral hepatic masses abut upon, but do not actually arch over it. In the 
post-larval wrasse a similar relation of the canal and the liver is noted. The prominent 
wrinkles of the gut in Anarrhichas disappear about the region of the pectoral fins, the 
canal forming a rounded thick-walled tube with one or two triangular folds. After passing 
the yolk-sac the canal is larger than in front, and transversely elongated in section, the 
mesenterial band fixing it to the roof of the abdominal cavity, though it is free elsewhere. 
Its contour is, however, broken superiorly by the wall of the large portal vessel. A com- 
paratively smooth portion of the gut follows, but folds again make their appearance, in 
the form of five or six prominent rugae in section. The mesentery dorsally is thickened, 
and is almost divided into two portions by a constriction — an upper rounded band, and 
a lower — which is thinned off superiorly. The ruga? now diminish, the mesentery disappears, 
and the urinary vesicle takes its place, while the anus opens externally. Proceeding 
backward, the epithelium of the gut is found to become finer and larger ; indeed, in section, 
posteriorly, it resembles that so characteristic of the alimentary wall in the Annelida ; 
then the folds reappear towards the rectum, and show a somewhat radial striation. 

The alimentary canal in the young salmon differs considerably from the foregoing in 
the region just behind the branchiae, since it forms a lax tube in transverse section, with 
thin walls greatly flattened from above downward. The wall increases in thickness in 
the region of the pectoral fins, and the circular coat assumes larger dimensions, so that 
the canal is less flattened. It is comparatively small for the size of the fish, the lumen 
being really smaller than that of the aorta. Towards the posterior part of the pectorals 
the gut is even less than in front of them. The epithelial (mucous) layer then begins 
to increase, and a folded condition of the gut causes two layers to appear in transverse 
section, a smaller superior and a larger inferior. In front of the liver the small calibre of 
the epithelial coat is in contrast with the thick circular (glandular) layer outside. 
In the hepatic region the lumen of the gut greatly enlarges, though it is still proportionally 
less than in the wolf-fish. Behind the liver it slightly diminishes in diameter, and again 
somewhat enlarges, before assuming the rounded condition characteristic of the rectum, 
the calibre of which is also smaller. The folds of the gut are much less prominent than 
in the wolf-fish ; and the inner surface of the large cylindrical cellular layer of the rectum 
is almost smooth. Some of these features, however, may partly be due to the state of the 
preparations. In the newly-hatched and living salmon, again, the alimentary canal appears 
between the yolk-sac and the anus as a greenish band. About the tenth day distinct 
transverse markings are observed in the tract, two especially conspicuous above the origin 
of the ventral fin. Between the fourth and fifth weeks, the functional activity of the 
alimentary canal is considerable, and numerous faecal masses occur in the rectum. The 
teeth are now evident in both upper and lower jaws. A little later food of various kinds 
is found in the stomach and intestine. The pyloric caeca on the nineteenth day form mere 
conical elevations on the duodenum, and have the aspect of short papillae of a cellulo- 
granular nature. It is remarkable that the pancreas (assuming the caeca to represent it) 


should have an origin so different in the higher animals. However, the condition of 
that organ in the sturgeon, tunny, and other forms would lend colour to such a view, 
even with the knowledge of the special rudiment in such forms as Salmo, Perca, and 

Towards the end of February, and later, many of the larval wolf-fishes showed a 
whitish streak in the interior of the intestine. It was uniformly opaque white, and 
apparently consisted of nutrient matter. In section this mass presented a series of 
peculiar crystalline and probably fatty bodies. 

The changes which take place in the structure of the digestive tract in Anarrhichas 
are noteworthy, and consist chiefly of the differentiation of the elements of the mucous 
lining, the increase of the circular and longitudinal muscular fibres, and the greater 
complexity of the folds of the walls, chiefly internally, but also externally. In the stage 
just described (17th January) the stomachal part of the canal has attained little develop- 
ment, and its mucous coat shows only a few frills of finely granular epithelium. As 
development proceeds, however, the oesophageal region of the canal is thrown into a 
complex series of frills, and the mucous lining is supplied with large globular glands. 
In the oldest stage (20th June) the folds of the oesophageal region are more complex 
than in the earlier stages, many of them being subdivided, and the longitudinal fibres 
(inside the circular) are more distinct dorsally and ventrally. The glands are arranged 
as a close and somewhat regular series of globular bodies along the inner surface of the 
folds (PI. XXVII. fig. 5). Proceeding backward the complexity of the folds increases, 
while the canal becomes rounder, the lamellae being pinnate in transverse section, from 
the number of the secondary folds. The globular glands now cease in the walls of the 
folded ridges, and an alteration occurs in the appearance of the latter, which assume a more 
or less circular condition in section, and in their wide bases are a series of large circular 
areolae,* probably glandular (dre, PI. XXVII. fig. 4), the inner wall of the gut over 
these being composed of closely-set cylindrical epithelial cells. These globular spaces 
also occur under the wall, where there are no lamellae. The calibre of the canal 
becomes smaller and the lamellae thicker, a few secondary processes or folds appear- 
ing on their surface. The latter has fine columnar epithelium, and the sub- 
mucous tissue is composed of granular glands, probably modifications of the large areolae 
in front. These lamellae become more distinctly pinnate before ceasing at the pylorus. 
The wall of the canal is highly muscular, the fibres forming a complexly interwoven layer 
externally. The next region of the canal to be distinguished is that behind the valvular 
folds of the former, and it is characterised by its thinner muscular walls, and the change 
in its glandular lining, for the numerous simple folds around its walls have coarser and 
more lax epithelium than the foregoing. Externally is a peritoneal investment (with 
probably a few muscular fibres), then longitudinal fibres grasped between the outer and 
the next layer, with, finally, an internal circular layer of fibres. 

Posteriorly, the gut diminishes in calibre, and by and by the folds chiefly affect the 

* These areolae in the sketch are perhaps too conspicuously cellular. 
VOL. XXXV. PART III. (NO. 19). 7 B 


upper wall. Certain parts of the rectum, on the other hand, are most complexly folded 
(PI. XXV. figs. 3, 4, 5, and 7), only a central area in the sections being devoid of rugae. 
The folds continue until the external aperture is reached (fig. 5). 

In the embryo of the salmon, forty days after fertilisation, the alimentary canal is 
closed in the cardiac region, and remains of small calibre throughout the rest of its 
extent; indeed, at the commencement of the segmental ducts, it is less in section than the 
notochord, and is very little larger than one of the segmental ducts. When thirteen days 
old, the alimentary canal in the region of the heart is more or less median in position, 
and presents two lateral slits. It then opens out, and again contracts to a small tube 
less than the girth of the notochord in the region of the liver, its epithelial coat forming 
a simple lining, without rugse. The tube, for the most part, remains simple to the 
posterior termination. 

The young salmon, forty-five days old, artificially reared, has its oesophageal region 
less developed as regards size, and the mucous rugae internally. The globular glands, 
however, have a similar arrangement, though they are proportionally larger. The 
basement-tissue is coarsely granular. The circular muscular coat is strongly developed. 
The canal is peculiar on account of the rapid narrowing that takes place, and the dis- 
appearance of the globular glands, the section of the entire canal in this region being- 
only a little larger than that of the notochord. 

In the Gadoids and Pleuronectids the structure of the alimentary canal agrees in the 
main with the features seen in Anarrhichas— the oesophageal portion, of small calibre, 
having thick walls with complex internal ridges, while posteriorly the walls become 
thinner and saccular, the internal ridges much less prominent, and the diameter of the 
canal greatly increased. In Callionymus, ^ inch long, the mucous membrane of the oral 
cavity rises into very thick prominences or longitudinal folds, which are richly glandular. 

Swim- Bladder. — In many post-larval fishes the swim-bladder retains, until a com- 
paratively late stage, its embryonic character as a sac lined by a thick layer of large soft 
epithelial cells, often so well developed anteriorly as to reduce the lumen of the organ to 
that of a narrow tube. In the cod, ^- inch in length, this is the condition, and the 
anterior portion is traversed by a large haemal trunk. The capacity of the bladder at a 
later stage enormously increases, and when the young Gadoid reaches the length of f 
inch, this structure has thin membranous walls with a layer of thickened epithelium 
only on the inner surface. In the post-larval Clupeoid, -^ inch long, it is large, and has 
a similar structure to that just described in the Gadoid, with, however, a less marked 
development of the epithelial coat. The epithelium of the inner surface in Callionymus, 
^ inch in length, rises into thick massive folds ; but the most remarkable development of 
this layer in the swim-bladder is seen in the gurnard, £ inch long, the large pulpy cells, 
each with its nucleus placed excentrically nearest the free surface, forming thick rugae, 
and supported by a layer of smaller cells irregularly scattered as an outer tunic. The 
large epithelial cells just described also occur in the swim-bladder of the Gadoid, 
when \ inch in length, and the deeply-stained nucleus is excentric in position ; but it 


is most distant from the free edge of the cell. In the later post-larval stages of the 
gurnard further changes occur, the deep glandular epithelium of the anterior part of the 
swim-bladder enclosing a narrow lumen. A little further back the thick glandular 
epithelium is continued into a lateral blastema of deeply stained tissue on each side. 
The cells are, however, altered in form, being spherical with a definite nucleus, and the 
cell-contents are now clear. The lumen of the bladder becomes posteriorly narrowed in the 
form of a neck, and pulpy globular cells, with deeply stained (glandular ?) contents take 
the place of the cubical epithelial cells of the fore part of the sac. These rounded cells 
form a superficially broad girdle. Behind the narrow neck the bladder expands again, but 
its walls are thinner and the layers distinguishable are fewer. Thus, in the anterior part, 
outside the greatly thickened mucous lining, a dense nucleated stratum occurs, which rests 
upon a striated fibrous layer, bounded externally by a very thin nucleated stratum — the 
nuclei being much flattened and elongated, and lying two or three deep. An outer tunic of 
thick connective-tissue circumscribes the bladder, and this rests in the anterior part upon 
the pigmented peritoneum. Posteriorly, the liver and intestine are in contact with the 
external connective tunic. The four layers just described, with the exception of the deep 
internal mucous layer, continue into the second part of the bladder, but are much thinner. 
The great bulk of the liver and the pronephric augmentation are probably influential 
in the shifting backward of the swim-bladder. As shown in the earlier part of this 
paper, the swim-bladder is a protrusion from the embryonic oesophagus ; but the 
lengthening of the gullet, the enlargement of the stomach and intestine, produce such 
changes in the disposition of the abdominal viscera as greatly to disturb the primitive 
relations of the various organs. Professor Cleland, in a valuable note* points out that 
the origin of the swim-bladder as a thoracic evagination must determine the regions 
of the alimentary tract (e.g., pharynx, stomach, and intestine) ; but it has to be borne in 
mind that, when the evagination takes place, the tract is very short, and the cystic 
duct is pushed out of the ventral wall in such close proximity to the outgrowth of the 
swim-bladder as to be included in the same section of the embryo, if cut in a slightly 
oblique plane, — the duodenum and pharynx in the early stage being separated by a very 
short interval. It is possible, therefore, that in the elongation and differentiation of the 
parts of the alimentary canal, the point of origin for the swim-bladder may, in post-larval 
and still more in adult stages of different species, be found in parts which do not perform 
the same physiological function. The position stated by Professor Cleland is not, how- 
ever, affected by this consideration, and the part called stomach in such a form as Clupea 
must be morphologically — if not functionally — pharyngeal. 

Liver. — The liver in Anarrhichas appears on both sides of the fish posteriorly, but 
in the salmon it is best seen on the right side. Consequently the arrangement of the 
blood-vessels which pass through it for the supply of the yolk- mass diverge considerably 
in the two species. The position of the liver is seen in the outlines of the right and left 
sides (PI. XX. figs. 2 and 4). 

* Memoirs in Anatomy, 1889. 


In the young Pleuronectid, -^ inch long, the liver is of considerable size, and fills up 
the main part of the large peritoneal cavity in front. This chamber is very capacious, 
and causes the abdomen to hang like a swollen sac on the ventral side of the young fish. 
Its walls are thin, a delicate sheet of muscle merely intervening between the integument 
and the internal peritoneal lining. Posteriorly the spacious intestine fills up the cavity. 
Anteriorly the narrow oesophagus passes above the liver; but in many forms it descends 
into the hepatic mass — so deeply in the salmon and in the wrasse -^ inch long as to be 
almost enveloped by the liver, as we see in a less degree in the Gadoid when -f inch in 

The gall-bladder forms a large rounded diverticulum in the midst of the liver on the 
right side. The outer layer is fibrous and probably muscular, while the inner consists of 
flattened, epithelial cells. At this stage two apertures are visible, one leading from the 
anterior division of the liver by a somewhat convoluted duct, the other entering the 
bladder towards its posterior border on the same side, i.e., the left. 

Spleen. — In the gurnard, \ inch long, and skulpin, ^ inch long, a rounded 
body occurs behind the liver. It is sheathed in a delicate cellular wall, and is formed 
of cells which stain deeply, grouped together in clumps, and forming a fairly solid 
organ. In Cottus scorpius, f- inch long, an organ of similar histological structure 
passes along the posterior dorsal surface of the liver. No limiting membrane separates 
it from the liver, but its cells stain more deeply than those of the hepatic mass. Its 
form is flattened and lobular, with blood-corpuscles filling up its interstices, and it is 
apparently the spleen. 

Branchial System. — On the first day the branchiae of the salmon have simple rows of 
papillae, each with a loop formed by a blood-vessel. On the second or third day, probably 
somewhat earlier, the operculum is noticed to flap actively, and a single series of blood- 
corpuscles rush up one side and down the other side of the branchial fimbriae. Small- 
celled cartilage is present in the branchiae on the eighth day, and respiration is active, 
while on the fourteenth day, the embryo respires 106 to 108 times per minute. On the 
nineteenth day (sixth to seventh week after fertilisation) the branchial fimbriae are pro- 
portionately large and blunt. 

In the wolf-fish each branchial arch, on the 16th January, presents externally the 
cuticular investment, then a dense cellular layer (hypoderm) of connective and glandular 
tissue, this being greatly thickened on the margin, which afterwards becomes laminated. 
The cartilages of the arch have in transverse section a thick marginal structureless ring 
and a cellular centre. The papillose processes in Anarrhichas, on the thickened edge, 
increase in size in February, but they do not show much further differentiation during 
this month. In March the branchial papillae are simple, and their vascularity is evident 
towards the end of the month, a large loop occurring in each. In April the elongated 
process becomes pinnate, a feature still more evident in May, so that each is feather-like, 
after the plan of those figured in the very young flounder (PI. XV. fig. 8). Little further 
change ensued in those examined in June. 


The arrangement of the hyoidean apparatus and of the branchial arches, in fairly 
developed forms, is shown in PI. XXV. figs. 1 and 2. 

The hyaline investment of the branchial arch described in the early larval Anarrhichas 
appears first in the post-larval Gadoid, -£% inch in length. It clothes the cartilaginous 
branchial arches and the hyomandibular element as a very thin perichondrial layer, which 
develops very strong blade-like ridges at a later stage (when the fish measures -§ inch 
in length). These ridges are three in number, one directed dorsally, the other two 
ventrally, and enclosing an angle in which the branchial vein passes. The central core 
of cartilage appears to be rapidly diminishing in diameter, and the hyaline investment is 
very thick. The branchial pinnae now form a double row along the artery of each arch, 
and they consist simply of folds of mucous membrane, the cells being very large and 
defined by a thin external membrane. Delicate cartilaginous supports appear as thin 
rods projecting from the arch, but not, however, as outgrowths from it. In the post-larval 
gurnard, T 5 ^- inch in length, these features are well marked, and in Cottus scorpius, -§ 
inch in length, the details are even more readily made out ; the hyaline deposit is not 
very thick, nor does it show any indication of the ridges described above. Outside the 
hyaline layer clothing the cartilaginous arch is an investing connective-tissue stratum in 
which the arterial and venous trunks lie. It appears to include some muscular elements. 
An epithelial layer lies externally, and it is much thickened on the anterior or upper side 
of the arch. Numerous large cells (mucous ?) and vesicles occur in it, and it forms the 
complex folds of the pinnae. In this form (Cottus) the cartilaginous rod developed in 
each pinna is very definite. In the gurnard, T \ inch in length, two muscles pass along 
a part of the first branchial arch on its ventral side, to be inserted on the copula (basi- 
branchial) of that arch. Before reaching the point of insertion the two muscles lie close 
together, and in the interval an upright plate of hard hyaline tissue is deposited ; 
anteriorly it sends out two horizontal plates from its ventral margin, and has in transverse 
section the form of an inverted T. The branchial artery is very well developed at 
this stage, having a dense external tunic, within which is a thick fibrous and muscular 
layer, with longitudinal fibres in separate bundles internally ; these cause ridges in the 
epithelial lining, and in transverse section they project boldly into the lumen of the 
vessel. In oblique sections along the branchial arches of a post-larval Gadoid, ^ 
inch in length, it is observed that the pinnae of one side of the arch are not placed oppo- 
site those of the other side, but alternately, each with the central cartilaginous support 
described above. 

Renal Organs. — Just behind the point where the notochord commences, and below 
the large basilar plate in the larval Anarrhichas of 17th January, cellular glands appear 
in the space between the parachordals and the roof of the pharynx. The glandular body 
on each side shows certain empty spaces around it, and lies beneath the auditory vesicle. 
It is composed of rather large nucleated cells, the whole having at first in vertical trans- 
verse section an ovoid outline and a thin hyaline investment. The jugular vein lies to 
the inner side of each, in the connective tissue which fills up the interval between the 


cranium and the pharyngeal roof below. Further back these glandular organs enlarge, 
and, as two preparations seem to show, they become lobulated, while the vessel on the 
inner side is surrounded by large nucleated cells. Pigment-corpuscles, moreover, appear 
at the side of the apertures and near the aorta. Toward the posterior border of the 
cranium, where a short conical spur abuts on the notochord on each side, the renal organ 
proper, or pronephros, commences in the form of numerous coils of the segmental duct 
(PI. XXIII. fig. 2, prn) ; moreover, shortly behind, in the middle line below the aorta, 
two symmetrical spaces occur, into each of which a round vascular mass (glomerulus) 
having a glandular appearance, projects, the outer side of each aperture being free 
(PI. XXVI. fig. 4, gl.). The pigment-corpuscles greatly increase, the main mass being 
situated dorsally over the segmental ducts, but also between and below them. Besides 
the ducts a quantity of glandular tissue (apparently Balfour and Parker's lymphatic 
tissue) occurs at the inner and inferior border of the region . The coils of the segmental 
duct form a considerable mass as they pass backward, but they soon diminish. The 
course of each, laterally, varies in those species of Teleosteans in which a swim-bladder 
is developed, the large capacity of this median structure causing the ducts to be widely 
separated anteriorly. In the post-larval cod, \ inch long, they pass along the side of the 
swim-bladder by a gently descending course to the urinary bladder. The increase in the 
cellular matter (which stains deeply) surrounding the tubules of the pronephros is very 
marked in the post-larval stages, and in the young gurnard, £ inch long, it forms quite a 
large lobate mass enclosed in a very delicate membrane or capsule, the two glomeruli being 
imbedded in this apparently glandular matrix. A similar development of these small 
cells, which stain deeply, is seen in the cod when T 5 ¥ of an inch in length, and much 
dense pigment is developed around the pronephros. The ducts, when traced back- 
ward at this stage, pass in the wolf-fish over the urinary bladder, and posteriorly 
curve round, and debouch into the bladder by a very sharp downward deflection. By 
and by only a single duct remains with a connective-tissue investment, in which pigment- 
corpuscles occur all round, except at the inner and inferior border. Subsequently a small 
solid mass of nucleated cells is seen at the inner and inferior border of the duct ; but no 
definite structure can be made out, and it soon comes to an end, to be succeeded by 
similar bands often passing toward the opposite duct. Moreover, in some sections a 
tubular structure indicates that these are probably segmental tubes in an early condi- 
tion. These sections are, however, situated high up — close by the sides of the aorta, 
but they do not appear to connect with it, and they have lax cellular walls. Soon the 
cardinal vein becomes single, and frequently the cells referred to form a thin band over 
it. Posteriorly the segmental ducts seem to diminish rather than increase in size, and 
the large cardinal vein widely separates them. By the diminution of the cardinal (caudal) 
vein the ducts again approach, and the space between the notochord and the rectum 
increases by the downward curve of the latter, while the fold of peritoneal mesentery, 
suspending the gut, disappears. The segmental ducts also move further from the 
notochord, and their lower ends merge into a dilated common part — the urinary vesicle — 


the wall of which superiorly appears to be composed of cylindrical epithelium with a fibrous 
outer investment passing into the connective tissue surrounding it (PL XXV. fig. 5). 

The glomerulus on 1st March presents a more distinctly looped arrangement (PI. 
XXVI. fig. 4), the basal regions being narrow and closely applied to each other, the 
free portion having a pear-shaped outline, and in transverse section showing long spaces, 
so that it has a somewhat looped appearance. These chambers above are formed by 
thin membranous walls studded over with small globular cells. The anterior cardinal 
veins, as they debouch into the venous sinus, are outside the glomerulus and the 
pronephros. On 16th March the cellular stroma towards the posterior part of the 
segmental ducts increases superiorly on the sides of the cardinal veins, and above the 
ducts. It soon forms two symmetrical masses above the latter, the constituent cells of 
which are arranged somewhat in rows, so that there is a tendency to a tubular structure. 
A series of vascular spaces, however, develop at the commencement of the urinary vesicle, 
and the tissue disappears. The preparations of 20th April distinctly show in this region 
segmental tubes on a miniature scale. 

These secondary growths, moreover, have extended much further forward. It is 
remarkable that, though the cellular tissue has greatly increased anteriorly, and for 
some distance backwards, no distinct tubes appear there at this date. 

On 1st May the chief change is the increase of pigment round the pronephros, which 
has a proportionally large bulk — the two sides forming on section an area equal to that 
of the alimentary canal. In the glomerulus nothing new is observed. A large vein 
runs down the right pronephros towards its termination, and then bends to the middle 
line. The segmental duct is now reduced to a single canal on each side, and, having 
reached the middle line beneath the aorta, the masses of cells superiorly, that is overlying 
the ducts, become more complex, and segmental tubes branch out — occurring both above 
and below the aorta, and beneath the cardinal vein in the middle line. The segmental 
ducts increase in size as the tubes become numerous, and each, like the ducts, has a 
hyaline investment (PI. XXVI. fig. 3). A granular substance occupies the centre of the 
segmental duct in section (PI. XXV. figs. 4, 5, 6, and 7). 

. The mass of cells, above described in Anarrhichas, is late in appearing in most of the 
other forms studied. In Labrus, T 7 ^ inch long, it forms a thick cylindrical column, 
over the two segmental ducts in the terminal part of their course. 

The intimate relation of the pronephros and the posterior cranial nerves is remarkable, 
the large cellular outgrowths of the brain, which mark the egress of the ninth and tenth 
nerves, are closely associated with the cellular stroma of the head-kidney, and the similarity, 
in the early condition, of the nervous and renal tissue is striking, especially at such a 
post-larval stage as that of the gurnard, when T 5 ^- of an inch long. The roots of the 
glosso-pharyngeal and vagus exhibit enormously enlarged ganglionic swellings in the 
example just instanced. The head-kidney, moreover, becomes so greatly increased in 
bulk dorso-ventrally as to extend in the cod, f inch in length — from the roof of the 
body-cavity almost to the level of the summit of the neural arch. 


The urinary vesicle presents a series of boldly marked folds superiorly, and its walls 
are in contrast with the massive sheath of the rectum. The segmental tubes are thus 
not developed in the pronephros, but have advanced considerably forward from the 
metanephros. In the pronephros the coils of the duct amidst the cellular mass seem to 
take the place of the tubes behind. Like the interior of the chamber, the duct leaving 
the urinary vesicle (PI. XXV. fig. 6) is lined with columnar epithelium, and it opens on a 
special papilla. The differences between the anterior region of the urinary vesicle before 
and after the development of the segmental tubes is clearly shown by contrasting figs. 5 
and 7 of PL XXV. The development of the pigment on the wall of the organ (urinary 
vesicle) is also a noteworthy feature. 

Body-Cavity. — The form and capacity of this chamber varies very much in post-larval 
stages. It may constitute, as pointed out on a prior page, a huge depending sac, especially 
well seen in Pleuronectids. In the flounder, ^ inch long, it is a thin-walled protruding 
pouch, anteriorly occupied by the bulky liver, while posteriorly the capacious intestine 
mainly fills up its cavity. Precisely the reverse condition obtains in the post-larval 
Clupeoid, ^ inch long, the body-cavity appearing merely as a slit-like space between the 
lengthened intestine and the peritoneum in front, the liver passing beneath the alimentary 
canal as well as the large swim-bladder behind, and reducing the space very much. In 
the goby, too, at ^ inch, the body-cavity diminishes posteriorly so much as to form 
an interval barely perceptible between the intestine and the body-wall. Its anterior 
end may, as in a Gadoid, T 5 ¥ inch long, pass beneath the pericardium, so that in section 
the ventricle of the heart occupies a position superior to the compact fore-end of the 
liver. This sub-pericardial protrusion of the body-cavity exhibits a thick muscular mass 
in its wall upon each side. The viscera in the post-larval wolf-fish and salmon appear to 
fill up the limited peritoneal chamber more completely than in the spacious and prominent 
cavity characteristic of such forms as the Pleuronectids and the ling. 



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VOL. XXXV. PART III. (NO. 19). 7 C 


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germinal cavity. 


pigmented layer of choroid. 






perichordal (mesoblastic) sheath. 








anterior commissure. 


haemal arch. 


pericardial chamber. 


anal fin. 






alar expansion of scutum (6s.). 




pectoral fin ; also pale commissural 




hyomandibular arch. 



aortic bulb. 


, , cleft. 


pectoral girdle. 




rudiment of heart. 




otocyst (early condition). 


hyoid element. 


pineal gland. 


auditory capsule. 

hyp. (or hy. ) hypoblast. 


primitive germinal cells. 




iter a tertio ad quartum ventri- 


, , opercular aperture. 





or ptg. palato-quadrate cartilage. 






also my. ) protovertebrse. 


blastodisk (before segmentation). 


interneural (spinous) cartilages. 


pronephric chamber. 


, , (after segmentation). 




primitive streak. 




subintestinal vein. 








,, opening into pre. 


urinary bladder formed by union of 


Kupffer's vesicle. 


peri vitelline space or " breathing 

segmental ducts. 









pituitary body. 


blastodermic rim. 


lower layer (primary entoderm) 




branchial arches. 



portal vein. 


,, clefts. 


lymphatic mass in front of kidney. 


parietal zone of blastoderm (resem- 


blastodermic shield. 


lobi inferiores. 

bling the Avian zona pellucida). 


carina, or ventral ridge of early 



quad, quadrate part of mandibular arch. 



meridional areas of protoplasmic 









canal (auditory, &c.) 




archinephric or segmental duct. 




permanent medullary canal. 


segmentation cavity. 

caud. caudal end of trunk. 


and mo. medulla oblongata. 


segmental tubes. 






sensory organs. 


caudal vein. 


membrana elastica externa. 


somatic mesoblast. 


cerebrum, anterior fore-brain. 




splanchnic mesoblast. 


cephalic end of embryo. 






caudal fin. 




spinal nerve. 


choroidal fissure. 




serous spaces around embryo. 




musculus retractor oris. 




cortical protoplasm. 




sinus venosus. 


cuticular (hypoblastic) notochordal 




thalamencephalon or posterior fore- 



myotomes, muscular somites, or 



cartilage, and connective tissue. 



lobes of roof of thalamencephalon. 


cornu trabecular. 




floor of thalamencephalic chamber. 


cardinal vein. 


neural arch. 




ductus choledochus. 




tori semicirculares. 


dorsal fin. 


notochordal sheath. 


,, longitudinales. 


dorsal or medullary groove. 


nervous cushion (auditory). 


urinary vesicle. 


ductus Cuvieri. 








neurenteric canal. 


ventricle (heart). 




cellular ring of notochord. 


ventral fins. 


embryonic (marginal) fin. 


nuclei of the periblast. 








ventricle (brain). 

ep. 2 

,, corneous. 


oleaginous sphere or globule. 


vesicle of thalamencephalon. 

'/'■ 1 



nasal pits. 




embryonic fin-rays. 


olfactory lobes. 


variously used for special structures. 


anterior transverse fibres. 


,, nerves. 






optic vesicle, also eye in later stages 


yolk-sac or embryonic membrane. 


roof of fore-brain 

(oc. in some figures). 


zona radiata. 


branchial rudiments. 
fornix of Gottsche. 


/■vr^rnv-iiil ii tyi 

UpC vifvi\>uiunit 

opem. optic commissure. 


Olfactory nerve. 


gut. fg, fore -gut. 


,, lobes. 


Optic nerve. 

my. mid-gut. 




Oculo-motor nerve. 

hg. hind-gut or rectum. 


valve-like flap of optic lobe 


Facial and auditory nerves. 






Trigeminal nerve. 




[The Plates were drawn under the impression that they would be published by the Fishery Board for Scotland. 
This was found to be impracticable. The larger share of the cost of their publication was borne by the Royal 
Society of Edinburgh, part of the balance being defrayed by a grant from the Fishmongers' Company of 

Plate I. 



























Fig. 13. 








Fig. 26. 

Ovum of Liparis montagui, pinkish in colour and with pitted capsule, 
Ovum of Coitus hubalis (?), showing boldly corrugated zona radiata, 
Ovum of Cottus scorpius, ....... 

Ovum of Liparis montagui, with pitted capsule. Adjacent ova are attached by strong 
facets, ......... 

Fertilised ovum of Clupea sprattus, ..... 

Ovum of Zeugopterus punctatus, in ripe (unfertilised) condition, 

Fertilised ovum of Trigla gurnardus, cells at the edge of the blastoderm, . 

T. gurnardus, pigment-corpuscles present at the closure of the blastopore, . 

,, pigment-corpuscles present at the closure of the blastopore, 

Spermatozoa of Trigla gurnardus, ..... 

Ova of Molva vulgaris, floating in calm water, showing a regular disposition 

next the central oil-globule indicates the blastoderm, 
0. ozglefinus, micropyle and accessory structure in oblique view, 

„ micropyle in profile, ...... 

a, external crater ; b, smaller internal orifice ; c, conical internal elevation. 
„ micropyle in profile, another view of a later condition in an ovum 4-£ hours 

after fertilisation, ....... 

„ micropyle in profile, 6 hours after fertilisation, . 

,, micropyle and accessory structure, oblique view, . . . . 

[The artist has placed the accessory structure above instead of at the side of the 
T. gurnardus, surface view of an abnormal zona radiata, . 

a, b, c, d, and e, successive changes observed in the polar nuclear body, apparently the 
union of male and female pronuclei in the ovum of T. gurnardus: a, at 25 mins. 
b, 30 mins.; c, 38 mins.; d, 46 mins.; e, side view of another similar body, 
Pleuronectes microcephalus, surface view of dotted and reticulated zona radiata, 
Trigla gurnardus, zona radiata with linear arrangement of dots, 
Pleuronectes platessa, zona radiata showing dotted structure, 
Liparis montagui, nearly ripe ovum, with reticulated capsule and oil-globule, 
Oblique view of the same. 
Agonus cataphractus, reticulated zona radiata, ..... 

Cyclopterus Imnpus, zona with dots of two sizes, ..... 

Anarrhichas lupus, zona in section, stained with hsematoxylin, which affects only a thin 
line a, . . . . . . . . 

Ovum of Solea vulgaris, viewed from above, so as to show the ring formed by the groups of 
small oil-globules ; the rim of the blastoderm is observed internally, and certain parasitic 
infusoria are seen externally, July 12, 1888, ...... 

x 50 

x 24 
x 24 

x 40 
x 56 
x 56 
x 90 
Highly magnified 
The ring 

x about 10 



x 75 




x 156 




x about 40 


Plate II. 

1. Pleuronectes flesus, vertical section through middle of blastoderm, 2nd day, April 7, 1886, x 175 

2. ,, ,, vertical section through middle of blastoderm, nearer the margin than 

in fig. 1, . . . . . . . x 175 
















Fig. 3. Gadiis ceglefinus, section through the blastoderm at the morula stage, 22nd hour, March 25, x 175 

1886, . . . . . . . . xl75 

„ ,, surface view of the edge of the blastoderm and periblast, 26th hour, March 

30, 1886, . . . . . . . x 175 

. „ ,, surface view of the same, 2 hours later (28th hour), . . x 175 

T. gurnardtis, yolk pitted by periblastic nuclei, viewed obliquely from above, . x 90 

„ periblast and edge of blastoderm, May 26, 1886, . . . . x 415 

,, similar view, 8th hour, June 25, 1886, . . . . x 156 

„ similar view, 8th hour, May 26, 1886, ..... x 415 

Gastrosteus spinachia, part of rim and of blastoderm when extended about nine-tenths over 

the yolk-surface, . . . . . . . . x 130 

Fig. 10. G. oegleftnus, section through the invaginated rim when almost at the equator, 4th day, and 

no germinal cavity present, ........ x 435 

Fig. 11. Ovum of Solea vulgaris, showing chromatophores over the yolk and groups of oil-globules 

segregated under the embryo, . . . . . . . x 50 

Fig. 12. Ovum of Gastrosteus spinachia in the morula condition, May 9, 1886 : x, facets of attach- 
ment to adjacent ova, . . . . . . . . x 25 

Fig. 13. Newly-hatched larva of Glupea sprattus, . . . . . . x 21 

Fig. 13a. Larva of Glupea sprattus, stage (in a somewhat abnormal specimen with minute papillae 
behind and below eye, and on the anterior surface of abdomen) more advanced than in 

the preceding figure, . . . . . . . . x 12 

Fig. 14. T. gumardus, May 24, 1886. Three oleaginous spheres were present in this ovum, . x 55 

Fig. 15. a, b, c, d, e, Pleuronectes flesus, vertical sections through the blastoderm on the 3rd day, 

April 7, 1886 : a, through the middle of the germ ; c through the marginal part, . x 150 

Fig. 16. Ovum of Gadus wglefinus, marginal blastodermic cells sending out filamentous protrusions 

under pressure, ......... x 230 

Fig. 17. Gadus aglefinus, vertical section through an ovum with the rim nearly at the equator; no 

germinal cavity appears, 4th day, March 24, 1885, . . . . x 64 

Fig. 18. Ovum of Gadus ceglefinus, vertical section through the blastoderm at the 2-cell stage; many 

vesicles are collected at the base of the blastomere, ..... x 435 

Fig. 19. Ovum of the haddock on the 3rd day, at 10 a.m., viewed from above, . . x 30 

Plate III. 

1. T. gumardus of the 3rd day, transverse section through the cephalic region, . . x 175 

of the 3rd day, transverse section posterior to fig. 1, . . . x 175 

of the 3rd day, transverse section through the terminal portion of trunk 

(primitive streak 1), ....... x 175 

of the 9th hour, May 28, 1886, cephalic end of embryo, . x 225 

of the 3rd day, May 27, 1886, snout of embryo, . . . x 250 

of the 72 nd hour, snout of embryo, ...... x 250 

of the 6th day, March 29, 1886 (Kupffer's vesicle and blastopore), . . x 400 

of April 26, 1886, 10 a.m., end view, . . . . x 250 

of April 26, 1886, somewhat later than fig. 8, end view of early embryo, . x 250 

of April 26, 1886, 11.40 a.m., . . . . . x 250 

of April 26, 1886, 12.45 noon, . . . . . x 250 

of May 1886, side view of blastopore and neurenteric canal, &c, . . x 97 

of May 1886, side view of blastopore and neurenteric canal, &c, . x 97 

of May 25, 1886, transverse section through the trunk (rim at equator), . x 175 

transverse section through the caudal region (primitive streak 1), same stage 

as fig. 11, . . . . . . x 175 

Fig. 13. „ of May 26, 1886, transverse section through the trunk (rim past equator), . x 175 

Fig. 14. G. morrhua, of April 26, 1886, Kupffer's vesicle viewed through the transparent embryo, x 150 


1. T. 















6. G. 






8. G. 


















11. G. 







Fig. 15. G. morrlma, of March 16, 1886, side view of blastopore with a plug of yolk, 

Fig. 16. T. gurnardus, of May 28, 1886, surface view of caudal end of embryo at an early stage, 

Fig. 17. G. ceglefinus, of March 25, 1886, 4 p.m., similar stage to the preceding figure, 

Fig. 18. G. morrhua, of April 26, 12.30 p.m., side view of caudal end, .... 

Fig. 19. „ front view of cephalic end of embryo, with peculiar fold of epiblast and 

hypoblast, ........ 

Fig. 20. „ similar view to fig. 18, . 

Fig. 21. „ of May 12, 1886, blastopore closing, ..... 

Fig. 22. T. gurnardus, of May 28, 1886, 4th day, caudal end of embryo viewed laterally, . 

Fig. 23. G. morrhua, of May 16, 1886, 90th hour, caudal end of embryo and blastopore viewed 

from above, ........ 

Plate IV. 






1 . Transverse section of embryo of Pleuronedes flesus, 4th day (snout), 










4th day (fore-brain), 
4th day (optic vesicles), 
4th day (nuchal region), 
4th day (posterior region of the trunk), 
4th day (posterior region of the trunk), 
4th day (posterior region of the trunk), 
4th day (posterior region of the trunk), 
4th day (close to margin of rim), 
4th day (close to margin of rim), 
immediately posterior to fig. 5b, 
the two layers of fig. 5e, 
the two layers of the blastodermic (extra 
embryonic) area, 

9. G. ceglefinus, 5th day, hind end of embryo, ..... 

10. ,, 5 th day, otocystic region, ...... 

11. T. gurnardus, 4th da)', longitudinal section, ..... 

12. „ 4th day, slightly oblique longitudinal vertical section (nuchal region), 

13. „ oblique section through fore-brain and eye, .... 

14. P. flesus, oblique longitudinal horizontal section of anterior end of embryo, 

15. G. ceglefinus, oblique longitudinal horizontal section of anterior end of embryo (rim at the 

equator of the egg), ...... 

15a. ,, oblique longitudinal horizontal section on a lower plane than fig. 15, 

16. „ 4th day, oblique longitudinal horizontal section, further advanced than 

fig- 15, . ' 

17. P. flesus, transverse section at a later stage than fig. 4, 

18. Molva vulgaris, horizontal section through the choroid fissure, 

19. ,, horizontal section through the choroid fissure at a lower plane than fig. 18, 

20. ,, transverse section through fore-brain, .... 






















Plate V. 

1. Molva vulgaris, front view of head, 6 th day, .... 

2. T. gurnardus, pigment-spots on the trunk, 4th day, June 1886, 

2a. ,, yolk-sac, 1st day out, ..... 

2b. „ pigment-spot on yolk-sac (three stages in progress of corpuscle), 

3. P. limanda, ovum with abnormal embryo, May 8, 1885, . 
3a. „ ovum with abnormal embryo, 3 days later, 

4. Undermined ovum (f) with oil-globule (034 inch in diam.), 

5. T. gurnardus, cyclopean embryo in ovo, ..... 

6. P. platessa, ovum with embryo well-developed, April 20, 1886, 


x about 55 
x 400 
x 40 
x 40 

7 D 



























Fig. 7. Molva vulgaris, aboral end of notochord, 10th day, 

Fig. 8. ,, ,, showing early pigment, 7th day, May 4, 1886, 

Fig. 9. „ „ embryo still further advanced, 8th day, May 5, 1886, 

Fig. 10. ,, ,, ventral aspect of embryo, .... 

Fig. 11. Pleuronectes limanda, ovum showing protoplasmic reticulations, 

Plate VI. 
G. morrliua, surface view of ear, May 10, 1886, ...... 

T. gurnardus, surface view of ear, 7th day, June 22, 1886, .... 

The anterior edge is on the right, and the dorsal is superior. 
„ longitudinal vertical section of the otocystic region; 3 days old, June 1, 1886, 

„ transverse section of the same region; 3 days old, June 1, 1886, 

G. aiglefinus, anterior end of larva, ....... 

Larva of T. gurnardus, from the dorsum, June 15, 1886, ..... 

Pleuronectes platessa, head of larva 2 days old, May 1886, .... Highly 

T. gurnardus, sensory organ in integument behind the cephalic region, May 12, 1886, 
„ sensory organ in integument behind the cephalic region, June 24, 1886, 

Transverse section through the otocystic region of larva of G. ceglefinus, 19 th day after 
hatching, ......... x 

Section of the same region, ........ 

Otolith of Cottus scorpius, -£% in. long (-188 inch), showing strongly stained core and un 
stained concentric stratum, ........ 

Plate VII. 

G. aiglefinus, transverse section, 13th day (7 hours before hatching), April 14, 1886, 
„ transverse section, 14th day after fertilisation (just emerged), 

,, transverse section, 3rd day after hatching, April 24, 1886, 

„ transverse section, posterior to fig. 3, 

„ transverse section through mid-gut and diverticulum (swim-bladder), 

„ transverse section, 2nd day out, April 24, 1886, 

,, transverse section (section succeeding fig. 6), ... 

„ longitudinal horizontal section, 3rd day out, March 23, 1886, 

Molva vulgaris, 6 days old, ventral aspect of the alimentary canal, 
T. gurnardus, longitudinal vertical section of the alimentary canal, 17th day, July 8, 1885 

„ dorsal view of pectoral fin of embryo before hatching, May 27, 1886, 

G. morrliua, section of the anal portion of the gut, 6th day out, May 4, 1886, 
Molva vulgaris, longitudinal horizontal section through the hind-gut, 2 days old, May 7 
1886, ...... 

„ „ longitudinal horizontal section at a lower plane, 

,, „ longitudinal horizontal section half-way down marginal fin 

„ „ longitudinal horizontal section of the anal opening, on a lower plane than 

fig- H, 

Plate VIII. 
Undetermined larva (d) with oil-globule (see p. 861), 

G. aiglefinus, 8th day after fertilisation, view of the heart, 8.50 p.m., March 30, 1886, 
T. gurnardus, from right side, focussed deeply: pew, anterior pericardial wall, July 18, 

„ ear, eye, and other organs, ....... 

„ 1st day, slightly oblique view of the cardiac region from below, June 1886, . 

Head and anterior region of T. gurnardus, newly hatched, .... 

Head of G. aiglefinus, 7th day, April 30, 1886, ...... 

Branchial region of T. gurnardus, newly hatched, ..... 


x 55 

x 70 

x about 30 

x about 40 





x 150 


t magnified 



about 300 












































, x 175 



x 175 




















x 25 

x 65 
x 65 
x 90 
















































Fig. 9. Heart and other organs of T. gurnardus, just hatched, seen from above, June 2, 1886, ! 

Fig. 10. Young pleuronectid (unknown sp.), April 7, 1887, front face view, 

Fig. 11. T. gurnardus, 4th day, transverse section through the heart, with mesoblast (mes), 

Plate IX. 

Anterior end of T. gurnardus, 3rd day, June 19, 1886, .... 

Molva vulgaris, 4 days old, May 12, 1886, 

„ „ 5 days old, May 13, 1886, ..... 

Head and anterior region of P. platessa, 8 mm. in length, and 4 days old, April 22, 1886, 
Anterior end of T. gurnardus, 8th day, June 24, 1886, .... 

G. ceglefinus, branchial and mandibular cartilages, April 20, 1886, 

Mandible of G. mglefinus, 10 days old, ...... 

Blastoderm of T. gurnardus at the stage of about sixty blastomeres : x, intra-blastomeric 
spaces apparently filled with fluid, ...... 

Molva vulgaris, margin of disc and nuclei of periblast, .... 

Gadus ceglefinus, 24th hour; the figure shows some marginal cells of the blastoderm, and a 

portion of the nucleated periblast, . . . . . . . x415 

Plate X. 

Slightly oblique view of the head of an advanced larva (15th day) of Trigla gurnardus, . x 40 

Trigla gurnardus, showing pigment in pectoral fin and visceral anatomy, 1 6th day, . Magnified 

,, „ ventral view of same, 16th day, ..... Magnified 

,, „ advanced embryo, 17th or 18th day, ..... Magnified 

Blastodisc of Trigla gurnardus at the 6 th hour, viewed from above ; third furrow nearly 

completed. Oil-globule (og) seen below, . . . . . x 80 

Larva of Gadus morrhua, ventral view of head, May 11, 1886, .... Magnified 

,, ,, ,, dorsal view of head, May 11, 1886 ; the heart is indicated by the 

dotted lines, ...... Magnified 

Opercular aperture of Molva vidgaris, May 10, 1886, ..... x 205 

Zona radiata of an abnormal egg of Solea vulgaris, showing flat papillse on the surface, . x 50 

Zona radiata of the pelagic egg with large perivitelline space with distinct punctures, . x 480 

Ovum of Gadus morrhua in the morula stage ; blastomeres boldly spherical, . . x 50 

Ovum of Pleuroneetes flesus, lateral view of the multicelled condition of the disc, . . x 35 

Plate XI. 

G. ceglefinus, transverse section through the fore-brain, 17th day, . . . . x 200 

,, transverse section through the fore part of heart, x 200 

,, transverse section posterior to fig. 2, . . . . . x 200 

„ transverse section through the fore part of the notochord, . . . x 200 

„ transverse section, lateral portion of section fig. 2, . . . x 435 

Molva vulgaris, transverse section, branchial region and heart, 11th day, . . . x 135 

,, ,, transverse section, branchial region and heart, 14th day, . . . x 135 

„ „ transverse section posterior to fig. 7, . . . . . x 135 

Gastrosteus spinachia, longitudinal vertical section through the branchial region about the 

time of hatching, . . . . . . x 135 

„ ,, longitudinal vertical section through the branchial region and oper- 
culum, slightly more advanced than fig. 9, . . . x 135 
G. aiglefinus, 2 days old, oblique horizontal section of the branchial region, . . x 175 
T. gurnardus, section of a portion of the protoplasmic investment of the oil-globule, . x 450 
Molva vulgaris, section through the oil-globule, showing pigment in the protoplasm, . x 150 
G. aiglefinus, 17 days old, transverse section through the hind-gut, . x 230 
„ transverse section through the base of the tail, . . . . x 230 











































Fig. 10. 

Fig. 11. 
Fig. 12. 
Fig. 13. 
Fig. 14. 
Fig. 15. 



Fig. 16. G. aiglefinus, transverse section showing a tract, probably sensory, in the lateral region of 
the tail, ........ 

Fig- 17. ,, transverse section near the tip of the caudal trunk, 

Fig. IS. T. gurnardus, 22 days old, bony elements in the roof of the mouth, probably palatines, 

Fig. 19. Clavicle of undetermined pleuronectid larva (possibly plaice), 

Fig. 20. T. gurnardus, 22 days old, premaxillary (?) elements : a, anterior extremity ; b, posterior. 

Plate XII. 
T. gurnardus, 1st day out, June 2, 1886, . 
G. morrhua, advanced larva, May 2, 1855, 
Early larva of Motdla mustela, May 8, 1886, 
Molva vulgaris, just hatched, 4.30 p.m., May 5, 1886, 
Cydopterus lumpus, newly-hatched, May 27, 1886, 
Larva of Pleuronedes flesus, 13 days old, April 26, 1886, 

,, „ ,,13 days old, dorsal view. 

Lateral view of the larva of P. platessa, May 7, 1886, . 

G. aiglefinus, 6 days old, longitudinal vertical section through pericardial chamber, sinus 
venosus, and branchial arches, ....... 






















x 70 


x about 32 

x 45 

x 60 

x 40 

. Magnified 

x 50 



Plate XIII. 

Fgi. 1. Larva of Liparis montagui, March 19, 1886, 

Fig. 2. Termination of the tail in the larva of Coitus scorpius, April 8, 1886, 

Fig. 3. Ovum of undetermined pleuronectid (?), with large perivitelline space. 

Fig. 4. Molva vulgaris, 1st day out, May 6, 1886, 

Fig. 5. Centronotus gunnellus, head and anterior region, March 18, 1886, . 

Fig. 6. „ ,, early larva, March 14, 1886, 

Fig. 6a. Caudal region of larval gunnel, ..... 

Fig. 7. Centronotus gunnellus, advanced larva, May 1, 1886, 

Plate XIV. 

Fig. 1. Embryo of G. ceglefinus, removed from capsule, April 1, 1886, viewed somewhat obliquely, 

Fig. 2. Head of larva of T. gurnardus, 3rd day, 

Fig. 3. Abnormal tail of larva of T. gurnardus, 1st day, June 7, 1886, 

Fig. 4. Anterior end of larva of Cottus scorpius (V), April 8, 1886, 

Fig. 5. P. platessa, anal region, ..... 

Fig. 6. Cardiac region of larva from ovum with large perivitelline space, 

Fig. 7. T. gurnardus, hind end of embryo and edge of blastopore, showing the adjoining nuclei of 

the periblast, ....... 

Fig. 8. ,, blastoderm shortly after the 6th hour, the 4th furrow in progress, . 

Plate XV. 

Molva vulgaris, subnotochordal trunks and blood-elements blc, May 8, 1886, 
Larva of Liparis montagui (?), showing vitelline circulation, April 12, 1886, 
Aboral end of the notochord and tail of post-larval P. flesus, 

Tip of the tail in the larval Motdla mustela, ..... 

5. Marginal fin and part of alimentary canal in the larva of Gastrosteus spinadtia, July 1 
1885 [the caudal fin in this figure has been marked ef instead of c/], 
Cydopterus lumpus, lateral view, 26 days old, June 17, 1885, 

Longitudinal section through the caudal portion of the notochord of G. aiglefinus, 13th day 
P. flesus, head of young specimen, May 18, 1886, ..... 
Cydopterus lumpus, same age as in fig. 6, ventral view showing sucker-like ventral fins, 



















x 24 

x 40 

x 40 

. x about 90 

. Magnified 

. x about 24 

. x about 24 

r, x 55 

x 55 


x 24 

x 55 

x 40 



x 80 


x 40 

x 55 



x 30 

j, x 200 

. Magnified 

x 25 




Plate XVI. 

Fig. 1. Pleuronectes flesus, larva 13 days old, April 26, 1886, 

Fig. 2. Gadus merlangus, early larva, April 24, 1885, .... 

Fig. 3. Pleuronectes limanda, 11 days old, May 22, 1886, .... 

Fig. 4. Dorsal view of the same, ....... 

Fig. 5. Slightly oblique view of an advanced larva of Pleuronectes platessa, May 7, 1886, 
Fig. 5a. Slightly oblique view of an advanced larva of ,, „ May 7, 1886 

aspect, ......... 

Fig. 6. Anterior end of the larva of P. limanda, 8th day after emerging, May 19, 1886, 
Fig. 7. Advanced larva of Liparis montagui, April 13, 1886, 
Fig. 8. Larva of Trigla gurnardus, 3rd day out, May 31, 1886, 
Fig. 9. Advanced larva of Cottus scorpius, April 13, 1886, 
Fig. 10. Abnormal ovum of Trigla gurnardus, July 8, 1885, 

Plate XVII. 

Gadus cegleflnus, larva, 7 days old, with circulation active, April 19, 1886, . 

Motella mustela, advanced larva, May 11, 1886, ...... 

Cyclopterus lurnpus, artificially extruded from the egg-capsule, . . . . 

Undetermined larva, with oil-globule, newly emerged from the ovum figured on Plate V. fig. 

Trigla gurnardus, post-larval stage, August 23, 1886, . 

,, „ post-larval stage, older stage, dorsal view, August 16, 1886, 

„ „ post-larval stage, older stage, side view, August 16, 1886, 

Young Gadus morrhua, June 11, 1886, ...... About 

Larva of Molva vidgaris on the 2nd day, May 8, 1886, 

Larva of „ „ 13 days old, May 13, 1886, 

Post-larval Cottus quadricornis, 

Advanced larval stage of Gadus merlangus, 

Newly-hatched larva of Solea vidgaris, 

Plate XVIII. 

Undetermined Pleuronectid, 24th hour after hatching, April 7, 1887, 

Another undetermined Pleuronectid, 2nd day out (for ovum, vide Plate XIII. fig. 3), 

Post-larval ling {Molva vulgaris), showing long ventral fins, 

Post-larval ling „ „ showing long ventral fins (later stage), 

Post-larval rockling {Motella), showing long ventral fins (later stage). The dorsal fin is 

entered in the text as df, 
Post-larval rockling {Motella), younger stage than fig. 5, 
Post-larval " witch " {Pleuronectes cynoglossus), 
Post-larval "witch" ,, ,, older stage, 

Advanced post-larval stage „ „ . . 

Advanced post-larval stage of armed bullhead {Agonus cataphr actus), April 28, 1887, 
Late larval stage of the same, April 4, 1887, ..... 

Plate XIX 

Young turbot {Rhombus maximus), August 23, 1886, 
2. Post-larval stage of Gadus morrhua, in spirit, May 1887, 

The same at a somewhat older stage than fig. 2, 

Young Cottus scorpius, May 6, 1887, 

Larval flounder {Pleuronectes flesus), 1st day out, . 

Post-larval stage of the angler {Lophius piscatorius), 

Larval haddock {Gadus cegleflnus), 1 clay old, ventral aspect, 
8. Larval cod {Gadus morrhua), 3rd day after hatching, 

(reduced) x 40 

x 40 

x 56 

x 50 

x 15 

































































x 18 
x 50 
x 18 
x 40 

x 40 

x 90 


4, x 40 

x 20 

. Enlarged 

. Enlarged 

natural size 


x about 75 
x 40 
x 5 
x 5 



x about 20 

x about 

x about 10 
x 6 
x 7 
x 7 
x 50 
x 6 
x 35 
Slightly larger than natural size 


Fig. 9. Larval ling (Molva vulgaris), 5th day, May 2, 1886, . . . . x 90 

Fig. 10. Larval example of undetermined Pleuronectid, obtained in St Andrews Bay 1887, . x 90 

Fig. 11. A young example of Callionymus lyra, 10 mm. in length, . . . . x 5 

Fig. 12. Ovum of Gadus morrhua in the morula condition; blastomeres somewhat rounded, . x 60 

Plate XX. 

Fig. 1. Anarrhichas lupus, larval tail showing radial striations, . . . . x 40 

Fig. 2. ,, ,, larva just emerged, and viewed from the left side, January 28, 1886; the 

great vitelline vein is indicated through the semi transparent yolk, . x about 12 
Fig. 3. Tail of the same at a more advanced stage, and when the blood-vessels form a fan, March 2, 

1886, ........... x 40 

Fig. 4. Larva of Anarrhichas lupus, just emerged, viewed from the right side, January 28, 1886, . x about 12 
Fig. 5. Dorsal view of a newly -hatched larva of the same species, January 1886, . . . x about 12 

Fig. 6. Group of firmly adherent ova of the wolf-fish {Anarrliichas lupus), with embryos far 

advanced, ........ Somewhat enlarged 

Fig. 7. Fresh ovum (unimpregnated) of the same species, .... Somewhat enlarged 

Fig. 8. Surface of lamina of the zona radiata in AnarrMchas, ..... x 500 

Fig. 9. Egg of salmon (Sahno salar) : a, seen from above ; b, viewed laterally, February 2, 

1862, ......... Somewhat enlarged 

Fig. 10. Egg of the same species, similarly viewed and magnified. 

Fig. 11. Portion of the zona radiata of the egg of the salmon, viewed as a transparent object, . x 300 

Fig. 12. Caudal region of the embryo of Gadus ceglefinus, 9th day after fertilisation, showing the 

neurenteric canal (?), . . . . . . . . x 70 

Fig. 13. Rectal region in the larva of Molva vulgaris, showing the communication of the urinary 

vesicle with the early rectal lumen, . . . . . . . x 70 

Plate XXI. 

Fig. 1. Anterior end of larval Anarrhichas lupus seen from the right side, February 16, 1886, . x 24 
Fig. 2. View of the circulation in the yolk-sac of the same species from the left side, February 23, 

1886, ........... x 44 

Fig. 3. Yolk-sac of more advanced larva of Anarrhichas, with oil-globule on the right side, May 1, 

1886, ........... x 18 

Fig. 3a. Yolk-sac of the same larva viewed from the left side, May 1, 1886, . . . x 18 

Fig. 4. Oblique view of the head of the larval Anarrhichas, January 1886, . . . x 24 
Fig. 5. Group of cells from the yolk-sac of the same form, ..... x about 650 

Fig. 5a.Altered blood-discs from larval Anarrhichas, ..... x about 650 

Fig. 6. Anterior end of segmental duct in the larval cod, May 10, 1886 ; reduced from a drawing x 200 
Fig. 7. Sensory bodies beneath the epiblast on the under surface of the snout of embryo of haddock, 

13th day after impregnation, ........ x 435 

Fig. 8. Horizontal section through the myotomes of the embryonic haddock on the 4th day after 

impregnation, .......... x 435- 

Plate XXII. 

Fig. 1. Solea vulgaris, ovum in the lenticular stage, and showing a few of the vesicles of the yolk 

under the periblast. The cells of the blastoderm for simplicity have been omitted, . x 50' 

Fig. 2. Tail of late larval Anarrhichas in April, the margin being now crenate, and the vessels well 

developed, . . . . . . . . . . x about 60 

Fig. 3. Vitelline circulation in the larval Anarrhichas on February 22, 1886, as seen from the right 

side, . . . • • . . . . . x 44 

Fig. 4. Salmon, 1 day old, ........ Slightly enlarged 

Fig. 5. Abnormal yolk-sac of salmon, ....... Slightly enlarged 

Fig. 6. Salmon, 1 week old, ....... Very slightly enlarged 

Fig. 7. Yolk-sac of the salmon with the oleaginous sphere at the tip ; an unusual position. 


Pig. 8. Salmon about a fortnight old. 

Pig. 9. Another example, slightly older, presenting differences in the outline of the yolk-sac and 

Pig. 10. Salmon, 1 month old, viewed from the dorsum. 
Fig. 11. Salmon, 5 weeks old, in profile. 
Pig. 12. Embryo of the gurnard (Triglot, gurnardus), showing early optic vesicles and myotomes, 

also compound vesicles anterior and posterior to Kupffer's vesicle, . . x 50 

Plate XXIII. 

Fig. 1 . Transverse vertical section (slightly oblique) of the head of the larval Anarrhichas, January 
23, 1886, through the optic lobes, optic thalami, and anterior lobes : ovf, valve-like 
flap of the optic lobes, . . . . . . . . x 40 

Fig. 2. Horizontal section through the anterior region of the same species, February 20, 1 886, show- 
ing the fourth ventricle, parachordals abutting on the oral end of notochord, pronephros, 
and other parts, . . . . . . . . x 40 

Fig. 3. Transverse vertical section through the fore-brain and the commencing optic lobes, the 
thalamencephalic chamber opening into the ventricle of the fore-brain beneath : flm, 
flap of mucous membrane at the side of the mouth, March 16, 1886, . . x 40 

Fig. 3a.Section somewhat behind the foregoing, showing the pineal gland and other parts, March 

16, 1886, . . . . . . . . . x 40 

Fig. 4. Transverse vertical section through the infundibulum and lobi inferiores : ms, muscles ; ct, 

connective tissue, June 20, 1886, . . . . . . x 97 

Fig. 5. Transverse vertical section through the pineal region, showing the posterior commissural 

bands of fibres, June 20, 1886, . . . . . . x 97 

Fig. 6. Transverse vertical (slightly oblique) section through the optic lobes and infundibulum of 

the same form, .......... Magnified 

Pig. 7. Transverse vertical section of tip of snout and through the nasal pits of the ling. The 

anterior epiblast is in contact with the hypoblast, . . . . x 135 

Fig. 8. Caudal extremity and Kupffer's vesicle of the embryo of Gadus ceglefinus, . . x 45 

Fig. 9. Embryo of Gadus a'glefinus, showing neurenteric canal, 9th day after fertilisation, . . x 45 

Fig. 10. Larva of Solea vulgaris, posterior aspect of the globular yolk, with large vesicles in proximity 

to the mass of oil-globules beneath the trunk, . . . . . x 40 

Plate XXIV. 

Fig. 1. Longitudinal vertical section of the head of the larval Anarrhichas nearly in the median 

line, January 23, 1886, . . . . . . . x 50 

Fig. 2. Similar section on one side of the median line, . . . . . x 40 

Fig. 3. Transverse vertical section of the head of the same form through the tori semicirculares and 

commencement of the notochord, January 16, 1886, . . . . x 52 

Pig. 4. Horizontal section through the anterior end of the fore-brain, with the olfactory bulbs 

(oil) and nerves (I.), June 20, 1886, . . . . . . x 52 

Pig. 5. Transverse vertical section of the brain of Anarrhichas, just behind the anterior commissure, 

traces of which are still present in the section, January 16, 1886, . . x 52 

Fig. 6. Similar section behind the former, showing the roof of the fore-brain, and the inferior 

fibres, fa, behind the optic tract, January 16, 1886, . . . . x 52 

Fig. 7. Circulation of the yolk-sac in the advanced larval Anarrhichas from the right side, March 

31, 1886, .......... Magnified 

Plate XXV. 

Pig. 1. Horizontal section through the heart and hyoid region of the larval Anarrhichas, March 29, 

1886 : chy, ceratohyal ; hhy, hypohyal, . . . . . x 60 


Fig. 2. Horizontal section through the branchial arches of the same form, April 20, 1886. The 

branchial arches are in